#1Main Street Capital Corporation MAINST CAPITAL CORPORATION Investor Presentation Second Quarter - 2020 min Filed Pursuant to Rule 433 Issuer Free Writing Prospectus dated August 7, 2020 Relating to Registration Statement No. 333-231146 NYSE: MAIN mainstcapital.com#2Disclaimers MAINST CAPITAL CORPORATION Main Street Capital Corporation (MAIN) cautions that statements in this presentation that are forward-looking, and provide other than historical information, involve risks and uncertainties that may impact our future results of operations. The forward-looking statements in this presentation are based on current conditions as of August 7, 2020, and include, but are not limited to, statements regarding our goals, beliefs, strategies, future operating results and cash flows, operating expenses, investment originations and performance, available capital, payment and the tax attributes of future dividends and shareholder returns. Although our management believes that the expectations reflected in any forward-looking statements are reasonable, we can give no assurance that those expectations will prove to have been correct. Those statements are made based on various underlying assumptions and are subject to numerous uncertainties and risks, including, without limitation: our continued effectiveness in raising, investing and managing capital; adverse changes in the economy generally or in the industries in which our portfolio companies operate; the potential impacts of the COVID-19 pandemic on our and our portfolio companies' business and operations, liquidity and access to capital, and on the U.S. and global economies, including public health requirements in response to the pandemic; changes in laws and regulations or business, political and/or regulatory conditions that may adversely impact our operations or the operations of our portfolio companies; the operating and financial performance of our portfolio companies and their access to capital; retention of key investment personnel; competitive factors; and such other factors described under the captions "Cautionary Statement Concerning Forward-Looking Statements," Management's Discussion and Analysis of Financial Condition and Results of Operations" and "Risk Factors" included in our filings with the Securities and Exchange Commission (www.sec.gov), including our most recent annual report on Form 10-K and quarterly report on Form 10-Q. We undertake no obligation to update the information contained herein to reflect subsequently occurring events or circumstances, except as required by applicable securities laws and regulations. MAIN has filed a registration statement (induding a prospectus and prospectus supplements) with the SEC for any offering to which this communication may relate and may file one or more supplements to the prospectus in the future. Before you invest in any of MAIN's securities, you should read the registration statement and the applicable prospectus and prospectus supplement(s) in order to fully understand all of the implications and risks of an offering of MAIN's securities. You should also read other documents MAIN has filed with the SEC for more complete information about MAIN and its securities offerings. You may get these documents for free by visiting EDGAR on the SEC website at www.sec.gov. Alternatively, MAIN will arrange to send you any applicable prospectus and prospectus supplement if you request such materials by calling us at (713) 350-6000. These materials are also made available, free of charge, on our website at www.mainstcapital.com. Information contained on our website is not incorporated by reference into this communication. The summary descriptions and other information included herein are intended only for informational purposes and convenient reference. The information contained herein is not intended to provide, and should not be relied upon for, accounting. legal or tax advice or investment recommendations. Before making an investment decision with respect to MAIN, investors are advised to carefully review an applicable prospectus to review the risk factors described or incorporated by reference therein, and to consult with their tax, financial, investment and legal advisors. These materials do not purport to be complete, and are qualified in their entirety by reference to the more detailed disdosures contained in an applicable prospectus and MAIN's related documentation. Main Street Capital Corporation NYSE: MAIN mainstcapital.com Page 2#3Main Street Capital Corporation Corporate Overview and Investment Strategy MAINST CAPITAL CORPORATION 2nd Quarter - 2020 Main Street Capital Corporation NYSE: MAIN mainstcapital.com Page 3#4MAIN is a Principal Investor in Private Debt and Equity Hybrid debt and equity investment strategy, internally managed operating structure and focus on Lower Middle Market differentiates MAIN from other investment firms MAINST CAPITAL CORPORATION Internally-managed Business Development Company (BDC) IPO in 2007 $4.0 billion in capital under management(1) - $3.0 billion internally at MAIN(1) - $1.0 billion as a sub-adviser to a third party(1) Primarily invests in the under-served Lower Middle Market (LMM) Targets companies with revenue between $10 million - $150 million; EBITDA between $3 million - $20 million Provides single source solutions including a combination of first lien, senior secured debt and equity financing Debt investments in Middle Market companies Issuances of first lien, senior secured and/or rated debt investments Larger companies than LMM investment strategy Debt investments originated in collaboration with other funds First lien, senior secured debt investments in privately held companies originated through strategic relationships with other investment funds Similar in size, structure and terms to LMM and Middle Market investments Attractive asset management advisory business Significant management ownership/ investment in MAIN Headquartered in Houston, Texas (1) Capital under management includes undrawn portion of debt capital as of June 30, 2020 Main Street Capital Corporation NYSE: MAIN mainstcapital.com Page +#5MAIN is a Principal Investor in Private Debt and Equity MAIN's unique investment strategy, efficient operating structure and conservative capitalization are designed to provide sustainable, long-term growth in recurring monthly dividends and long-term capital appreciation to our shareholders MAINST CAPITAL CORPORATION Long-term focus on delivering our shareholders sustainable growth in net asset value and recurring dividends per share Consistent cash dividend yield-dividends paid monthly MAIN has never decreased its monthly dividend rate 86% increase in monthly dividends from $0.33 per share paid in Q4 2007 to declared dividends of $0.615 per share for Q4 2020 Owns three Small Business Investment Company (SBIC) Funds Main Street Mezzanine Fund (2002 vintage), Main Street Capital II (2006 vintage) and Main Street Capital III (2016 vintage) + Provides access to 10-year, low cost, fixed rate government- backed leverage Strong capitalization and liquidity position-stable, long-term debt and significant available liquidity to take advantage of opportunities Favorable opportunities in capital markets through investment grade rating of BBB-/Stable from Standard & Poor's Rating Services • Total SBIC debenture regulatory financing capacity of $350.0 million Main Street Capital Corporation NYSE: MAIN mainstcapital.com Page 5#6MAIN is a Principal Investor in Private Debt and Equity Focus on LMM equity investments and efficient operating structure differentiates MAIN and provides opportunity for significant total returns for our shareholders MAINST CAPITAL CORPORATION Equity investments in LMM portfolio provide both the opportunity to grow net asset value (NAV) per share and generate recurring dividend income and periodic realized gains to support MAIN's dividend growth " " NAV growth of $8.00 per share (or 62%) since 2007 Cumulative net realized gains from LMM portfolio investments of $117.1 million ($20.4 million net for the total investment portfolio) since the Initial Public Offering Approximately $2.37 per share in cumulative, pre-tax net unrealized appreciation on LMM portfolio at June 30, 2020 • Realized gains provide taxable income in excess of net investment income and help fund MAIN's dividends Internally managed operating structure provides significant operating leverage Favorable ratio of total operating expenses, excluding interest expense, to average total assets of approximately 1.2% (1) Greater portion of gross portfolio returns are delivered to our shareholders Significant positive impact to Net Investment Income Alignment of interests between MAIN management and our shareholders (1) Based upon the trailing twelve months ended June 30, 2020 Main Street Capital Corporation NYSE: MAIN mainstcapital.com Page 6#7MAINST CAPITAL CORPORATION MAIN Strategy Produces Differentiated Returns Enhanced Value Proposition - Three Ways to Win are Better Than One 1. Sustain and Grow Dividends Efficient operating structure provides operating leverage to grow distributable net investment income, and dividends paid, as investment portfolio and total investment income grow 86% increase in monthly dividends from $0.33 per share paid in Q4 2007 to declared dividends of $0.615 per share for Q4 2020 Never decreased regular monthly dividends (including through 2008/2009 recession) Paid or declared $29.600 per share in total dividends since October 2007 IPO at $15.00 per share ($25.560 per share in regular dividends and $4.040 per share in supplemental dividends) Multi-faceted investment strategy supports growth of dividends over various cycles and markets 2. Meaningfully Grow Net Asset Value ("NAV") Per Share $12.85 at December 31, 2007 to $20.85 at June 30, 2020-62% growth; CAGR of 3.9% Represents incremental economic return to investors beyond dividends MAIN's debt-focused peers (which comprises most BDCs) cannot generate NAV per share growth through the cycles Unrealized appreciation is a good proxy for future dividend growth without the need for additional capital through growing portfolio dividend income and harvested realized gains from equity investments Ability to grow NAV per share provides opportunity for MAIN stock share price appreciation and additional shareholder returns 3. Supplement Growth in Distributable Net Investment Income with Periodic Realized Gains LMM equity component of investment strategy provides opportunity for meaningful realized gains (analogous to PIK income on debt investments from cash flow perspective, but more tax efficient and without cap on upside) Realized gains validate the quality of MAIN's unrealized appreciation Realized gains can be paid to shareholders as dividends or retained for future reinvestment due to MAIN's unique tax structure Main Street Capital Corporation NYSE: MAIN mainstcapital.com Page 7#8Historical Monthly Dividends, Net Asset Value ("NAV") and Distributable Net Investment Income ("DNII") (1) Per Share MAIN's unique focus on equity investments in the Lower Middle Market provides the opportunity for significant NAV per share growth MAIN's efficient operating structure provides significant operating leverage, greater dividends and greater overall returns for our shareholders DNII and Dividends Per Share $0.80 I Recessionary $0.70 T Period $0.60 T $0.50 $0.40 $0.30 $0.20 T MAINST CAPITAL CORPORATION 2007 2009 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 $26.00 $24.00 $22.00 $20.00 $18.00 $16.00 $14.00 $12.00 $10.00 $0.00 Monthly Dividends -DNII per share ( NAV per share • In addition to monthly dividends above, $4.04 per share of supplemental dividends have been paid since 2013 ⚫ Annual return on equity averaging approximately 12.5% from 2010 through the second quarter of 2020 (1) See Non-GAAP Information included on page 47 of this presentation. Main Street Capital Corporation NYSE: MAIN mainstcapital.com Page 8 NAV Per Share#9Lower Middle Market (LMM) Investment Strategy LMM investment strategy differentiates MAIN from its competitors and provides attractive risk- adjusted returns Investment Objectives MAINST CAPITAL CORPORATION High cash yield from secured debt investments (10.9% weighted- average cash coupon as of June 30, 2020); plus Dividend income and periodic capital gains from equity investments Investments are structured for (i) protection of capital, (ii) high recurring income and (iii) meaningful capital gain opportunity • Focus on self-sponsored, "one stop" financing opportunities Partner with business owners and entrepreneurs Recapitalization, buyout, growth and acquisition capital • Extensive network of grass roots referral sources Strong and growing "Main Street" brand recognition/reputation Provide customized financing solutions Investments have low correlation to the broader debt and equity markets and attractive risk-adjusted returns Main Street Capital Corporation NYSE: MAIN mainstcapital.com Page 9#10MAINST CAPITAL CORPORATION LMM Investment Opportunity MAIN targets LMM investments in established, profitable companies Characteristics of LMM provide beneficial risk- reward investment opportunities Large and critical portion of U.S. economy 175,000+ domestic LMM businesses(1) LMM is under-served from a capital perspective and less competitive Inefficient asset class generates pricing inefficiencies Typical entry enterprise values between 4.5X-6.5X EBITDA Typical entry leverage multiples between 2.0X-4.0X EBITDAto MAIN debt investment Partnerrelationship with the management teams of our portfolio companies vs a "commoditized vendor of capital" (1) Source: U.S. Census 2012 - U.S. Data Table by Enterprise Receipt Size; 2012 County Business Patterns and 2012 Economic Census; includes Number of Firms with Enterprise Receipt Size between $10,000,000 and $99,999,999 Main Street Capital Corporation NYSE: MAIN mainstcapital.com Page 10#11MAINST CAPITAL CORPORATION Private Loan Investment Strategy Private Loan portfolio investments are primarily debt investments in privately held companies which have been originated through strategic relationships with other investment funds on a collaborative basis, and are often referred to in the debt markets as "club deals" Investment Objectives Access proprietary investments with attractive risk-adjusted return characteristics ⚫ Generate cash yield to support MAIN monthly dividend Investment Characteristics • Investments in companies that are consistent with the size of companies in our LMM and Middle Market portfolios Proprietary investments originated through strategic relationships with other investment funds on a collaborative basis • Current Private Loan portfolio companies have weighted-average EBITDA of approximately $51.8 million(1) Investments in secured debt investments First lien, senior secured debt investments Floating rate debt investments 8% -12% targeted gross yields • Weighted-average effective yield (2) of 8.7% (3) Net returns positively impacted by lower overhead requirements and modest use of leverage Floating rate debt investments provide matching with MAIN's floating rate credit facility (1) This calculation excludes four Private Loan portfolio companies as EBITDA is not a meaningful metric for these portfolio companies (2) Weighted-average effective yield includes amortization of deferred debt origination fees and accretion of original issue discount, but excludes fees payable upon repayment of the debt instruments and any debt investments on non-accrual status (3) Weighted-average effective yield is calculated using the applicable floating interest rate as of June 30, 2020 Main Street Capital Corporation NYSE: MAIN mainstcapital.com Page 11#12MAINST CAPITAL CORPORATION Middle Market Debt Investment Strategy MAIN maintains a portfolio of debt investments in Middle Market companies Investment Objective Generate cash yield to support MAIN monthly dividend Investments in secured and/or rated debt investments First lien, senior secured debt investments Floating rate debt investments Larger companies than the LMM investment strategy ⚫ Current Middle Market portfolio companies have weighted-average EBITDA of approximately $78.1 million(1) Large and critical portion of U.S. economy Nearly 200,000 domestic Middle Market businesses(2) More relative liquidity than LMM investments 6%-10% targeted gross yields • Weighted-average effective yield(3) of 7.7% (4) Net returns positively impacted by lower overhead requirements and modest use of leverage Floating rate debt investments provide matching with MAIN's floating rate credit facility (1) This calculation excludes two Middle Market portfolio companies as EBITDA is not a meaningful metric for these portfolio companies (2) Source: National Center for The Middle Market; includes number of U.S. domestic businesses with revenues between $10 million and $1 billion (3) Weighted-average effective yield includes amortization of deferred debt origination fees and accretion of original issue discount, but excludes fees payable upon repayment of the debt instruments and any debt investments on non-accrual status (4) Weighted-average effective yield is calculated using the applicable floating interest rate as of June 30, 2020 Main Street Capital Corporation NYSE: MAIN mainstcapital.com Page 12#13MAINST CAPITAL CORPORATION Asset Management Business MAIN's asset management business represents additional income diversification and the opportunity for greater shareholder returns MAIN's internally managed operating structure provides MAIN's shareholders the benefits of this asset management business In May 2012, MAIN(1) entered into an investment sub-advisory agreement with the investment adviser to HMS Income Fund, Inc. ("HMS"), a non-listed BDC MAIN(1) provides asset management services, including sourcing, diligence and post-investment monitoring MAIN(1) receives 50% of the investment adviser's base management fee and incentive fees -MAIN() base management fee-1% of total assets - MAIN incentive fees - 10% of net investment income above a hurdle and 10% of net realized capital gains Benefits to MAIN • No significant increases to MAIN's operating costs to provide services (utilize existing infrastructure and leverage fixed costs) No invested capital - monetizing the value of MAIN franchise Significant positive impact on MAIN's financial results - $2.2 million contribution to net investment income in the second quarter of 2020 $4.5 million contribution to net investment income in the six months ended June 30, 2020 (2) $11.7 million contribution to net investment income for the year ended December 31, 2019) $69.1 million of cumulative unrealized appreciation as of June 30, 2020 In June 2020, MAIN(1) entered into an asset purchase agreement with the investment adviser to HMS to become the sole investment adviser to HMS Transaction closing is subject to HMS shareholder approval and other customary closing conditions • MAIN(1) will receive 100% of the management fees following transaction close Base management fee to be reduced from 2.0% to 1.75% of total assets No change to incentive fee calculation (1) Through MAIN's wholly owned unconsolidated subsidiary, MSC Adviser, LLC (2) Contribution to Net Investment Income includes (a) dividend income received by MAIN from MSC Adviser I, LLC and (b) operating expenses allocated from MAIN to MSC Adviser I, LLC Main Street Capital Corporation NYSE: MAIN mainstcapital.com Page 13#14MAINST CAPITAL CORPORATION MAIN Regulatory Framework Highly regulated structure provides significant advantages and protections to our shareholders, including investment transparency, tax efficiency and beneficial leverage Operates as a Business Development Company Regulated by Securities and Exchange Commission - 1940 Act Publicly-traded, private investment company Regulated Investment Company (RIC) tax structure Eliminates corporate level income tax " • . Efficient tax structure providing high yield to investors Passes through capital gains to investors Small Business Investment Company (SBIC) subsidiaries Regulated by the U.S. Small Business Administration (SBA) Access to low cost, fixed rate, long-term leverage • Total SBIC debenture regulatory financing capacity of $350.0 million • " Total outstanding leverage of $314.8 million through two wholly owned SBIC Funds MAIN is a previous SBIC of the Year Award recipient Main Street Capital Corporation NYSE: MAIN mainstcapital.com Page 14#15MAIN Corporate Structure - Internally Managed "Internally managed" structure means no external management fees or expenses are paid, providing operating leverage to MAIN'S business; MAIN targets total operating and administrative costs at or less than 2% of assets Main Street Capital Corporation (BDC/RIC) Assets: $1,969 million Line of Credit: $315 million ($740.0 million facility)(¹) Notes: $510 million(2) MAINST CAPITAL CORPORATION Main Street Mezzanine Fund, LP (2002 vintage SBIC) Assets: SBIC Debt: Main Street Capital II, LP (2006 vintage SBIC) $214 million $140 million outstanding Assets: $82 million Main Street Capital III, LP (2016 vintage SBIC) Assets: $281 million SBIC Debt: $175 million outstanding (1) As of June 30, 2020, MAIN's credit facility had $740.0 million in total commitments. MAIN's credit facility includes an accordion feature which could increase total commitments up to $800.0 million (2) $325.0 million of 5.20% Notes due May 2024 and $185.0 million of 4.50% Notes due December 2022. In July 2020, MAIN issued an additional $125.0 million aggregate principal amount of the 5.20% Notes, resulting in an outstanding principal balance of $450.0 million for the 5.20% Notes. Main Street Capital Corporation NYSE: MAIN mainstcapital.com Page 15#16MAINST MAIN Co-Founders and Executive Management Team Dwayne Hyzak; CPA CEO David Magdol President and CIO Vince Foster; CPA & JD Executive Chairman Jesse Morris; CPA COO and Executive Vice President Brent Smith; CPA CFO and Treasurer Jason Beauvais; JD SVP, GC, CCO) and Secretary CAPITAL CORPORATION Co-founded MAIN; Joined Main Street group in 2002; affiliated with Main Street group since 1999 Director of acquisitions/integration with Quanta Services (NYSE: PWR) Manager with a Big 5 Accounting Firm's audit and transaction services groups Co-founded MAIN; Joined Main Street group in 2002 Vice President in Lazard Freres Investment Banking Division Vice President of McMullen Group (John J. McMullen's Family Office) • Co-founded MAIN and MAIN predecessor funds (1997) " • Co-founded Quanta Services (NYSE: PWR) Partner in charge of a Big 5 Accounting Firm's Corporate Finance/Mergers and Acquisitions practice for the Southwest United States • Joined MAIN in 2019 Previously Executive Vice President with Quanta Services (NYSE: PWR) Prior experience with a Big 5 Accounting Firm and a publicly-traded foodservice distribution company • Joined MAIN in 2014 Previously CFO with a publicly-traded oilfield services company Prior experience with a Big 5 Accounting Firm and a publicly-traded financial consulting firm ■ Joined MAIN in 2008 Previously attorney for Occidental Petroleum Corporation (NYSE: OXY) and associate in the corporate and securities section at Baker Botts LLP (1) Member of MAIN Executive Committee (2) Member of MAIN Investment Committee (3) Chief Investment Officer (4) Chief Operating Officer (5) Chief Compliance Officer Main Street Capital Corporation NYSE: MAIN mainstcapital.com Page 16#17Monthly Dividends Per Share - Sustainable Growth Recurring monthly dividend has never been decreased and has shown meaningful (86%) growth since IPO Based upon the current annualized monthly Trailing Twelve Months Monthly Dividends Per Share $2.50 $2.25 MAINST CAPITAL CORPORATION dividends for the fourth $2.00 quarter of 2020, the annual effective yield on MAIN'S $1.75 stock is 7.9% (1) MAIN has also paid $4.040 in supplemental dividends $1.50 Cumulative dividends paid or declared, including supplemental dividends, from October 2007 IPO (at $15.00 per share) through Q4 2020 equal $29.600 per share(2) $1.25 $1.00 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 Monthly Dividends (2) (1) As of August 5, 2020; based upon the closing market price of $31.05 per share and the annualized most recently declared monthly dividends (2) Based upon dividends which have been paid or declared as of August 5, 2020 Main Street Capital Corporation NYSE: MAIN mainstcapital.com Page 17#18Main Street Capital Corporation Investment Portfolio MAINST CAPITAL CORPORATION 2nd Quarter - 2020 Main Street Capital Corporation NYSE: MAIN mainstcapital.com Page 18#19MAINST CAPITAL CORPORATION Total Investment Portfolio Diversity provides structural protection to investment portfolio, revenue sources, income, cash flows and shareholder dividends Includes complementary LMM debt and equity investments, Private Loan debt investments and Middle Market debt investments Total investment portfolio at fair value consists of approximately 49% LMM / 27% Private Loan / 17% Middle Market / 7% Other (1) Portfolio investments 177 LMM, Private Loan and Middle Market portfolio companies Average investment size of $13.0 million(2) • . Largest individual portfolio company represents 3.7% (3) of total investment income and 3.0% of total portfolio fair value (most investments are less than 1%) Eleven non-accrual investments, which represent 1.9% of the total investment portfolio at fair value and 6.3% at cost. Weighted-average effective yield(4) of 9.5% Significant diversification Issuer • Industry Transaction type • Geography ⚫ End markets . Vintage (1) Other includes MSC Adviser, LLC, MAIN'S External Investment Manager (2) As of June 30, 2020; based on cost (3) Based upon total investment income for the trailing twelve month period ended June 30, 2020 (4) Weighted-average effective yield includes amortization of deferred debt origination fees and accretion of original issue discount, but excludes fees payable upon repayment of the debt instruments and any debt investments on non-accrual status Main Street Capital Corporation NYSE: MAIN mainstcapital.com Page 19#20Total Portfolio by Industry (as a Percentage of Cost) (1) MAINST CAPITAL CORPORATION Machinery, 8% Aerospace & Defense, 5% ■ Professional Services, 5% Construction & Engineering, 5% ■ Media, 4% ■Diversified Telecommunication Services, 4% ■ Hotels, Restaurants & Leisure, 4% Electronic Equipment, Instruments & Components, 3% ■ Oil, Gas & Consumable Fuels, 3% Food Products, 3% ■ Diversified Financial Services, 2% Computers & Peripherals, 1% ■ Diversified Consumer Services, 1% ■Food & Staples Retailing, 1% Intemet & Catalog Retail, 1% ■Commercial Services & Supplies, 5% Energy Equipment & Services, 5% ■ Health Care Providers & Services, 5% Intemet Software & Services, 4% IT Services, 4% ■Leisure Equipment & Products, 4% ■Software, 4% ■Communications Equipment, 3% Specialty Retail, 3% Distributors, 3% ■Containers & Packaging, 2% ■Trading Companies & Distributors, 1% ■Transportation Infrastructure, 1% Chemicals, 1% Other, 5% (1) Excluding MAIN's Other Portfolio investments and the External Investment Manager, as described in MAIN's public filings, which represent approximately 5% of the total portfolio Main Street Capital Corporation NYSE: MAIN mainstcapital.com Page 20#21Diversified Total Portfolio (as a Percentage of Cost) (1) MAINST CAPITAL CORPORATION Invested Capital by Transaction Type LBO/MBO 39% 12% 4% 45% Acquisition Growth Capital Recapitalization/ Refinancing Invested Capital by Geography (2) 22% 20% 19% 13% 26% (1) Excluding MAIN'S Other Portfolio investments and the External Investment Manager, as described in MAIN's public filings, which represent approximately 5% of the totall portfolio (2) Based upon portfolio company headquarters and excluding any MAIN investments headquartered outside the U.S., which represent approximately 2% of the total portfolio Main Street Capital Corporation NYSE: MAIN mainstcapital.com Page 21#22MAINST CAPITAL CORPORATION LMM Investment Portfolio LMM Investment Portfolio consists of a diversified mix of secured debt and lower cost basis equity investments 69 portfolio companies/$1,188.0 million in fair value • 49% of total investment portfolio at fair value Debt yielding 11.6% (1) (65% of LMM portfolio at cost) + • 98% of debt investments have first lien position 65% of debt investments earn fixed-rate interest Approximately 780 basis point net cash interest margin vs "matched" fixed interest rate on SBIC debentures Equity in 99% of LMM portfolio companies representing 41% average ownership position (35% of LMM portfolio at cost) Opportunity for fair value appreciation, capital gains and cash dividend income • 62% of LMM companies (2) with direct equity investment are currently paying dividends Fair value appreciation of equity investments supports Net Asset Value per share growth Lower entry multiple valuations, lower cost basis $155.7 million, or $2.37 per share, of cumulative pre-tax net unrealized appreciation at June 30, 2020 (1) Weighted-average effective yield includes amortization of deferred debt origination fees and accretion of original issue discount, but excludes fees payable upon repayment of the debt instruments and any debt investments on non-accrual status (2) Includes the LMM companies which (a) MAIN is invested in direct equity and (b) are treated as flow-through entities for tax purposes; based upon dividend income for the trailing twelve month period ended June 30, 2020 Main Street Capital Corporation NYSE: MAIN mainstcapital.com Page 22#23LMM Investment Portfolio MAINST CAPITAL CORPORATION LMM Investment Portfolio is a pool of high quality, seasoned assets with attractive risk-adjusted return characteristics Median LMM portfolio credit statistics: Senior leverage of 2.7x EBITDA through MAIN debt position 2.8x EBITDA to senior interest coverage • Total leverage of 3.0x EBITDA including debt junior in priority to MAIN • Free cash flow de-leveraging improves credit metrics and increases equity appreciation Average investment size of $17.2 million at fair value or $15.0 million on a cost basis (less than 1% of total investment portfolio) Opportunistic, selective posture toward new investment activity over the economic cycle High quality, seasoned LMM portfolio Total LMM portfolio investments at fair value equals 115% of cost Equity component of LMM portfolio at fair value equals 158% of cost Significant portion of LMM portfolio has de-leveraged and a majority of the LMM portfolio investments have experienced equity appreciation Main Street Capital Corporation NYSE: MAIN mainstcapital.com Page 23#24LMM Portfolio by Industry (as a Percentage of Cost) MAINST CAPITAL CORPORATION ■Machinery, 10% Energy Equipment & Services, 8% Food Products, 6% Leisure Equipment & Products, 5% Internet Software & Services, 4% Hotels, Restaurants & Leisure, 4% ■Media, 3% ■Distributors, 3% ■Diversified Telecommunication Services, 2% Specialty Retail, 2% ■Diversified Financial Services, 2% Electrical Equipment, 1% ■Professional Services, 10% ■ Electronic Equipment, Instruments & Components, 7% ■Construction & Engineering, 6% ■Software, 5% ■Containers & Packaging, 4% ■Commercial Services & Supplies, 4% ■Computers & Peripherals, 3% ■Diversified Consumer Services, 2% ■ Building Products, 2% ■IT Services, 2% ■Health Care Providers & Services, 1% ■ Other, 4% Main Street Capital Corporation NYSE: MAIN mainstcapital.com Page 24#25Diversified LMM Portfolio (as a Percentage of Cost) Invested Capital by Transaction Type Invested Capital by Geography (1) MAINST CAPITAL CORPORATION LBO/MBO Acquisition Recapitalization/ Refinancing 23% 19% 14% 38% 52% 4% 6% (1) Based upon portfolio company headquarters Main Street Capital Corporation Growth Capital 11% 33% NYSE: MAIN mainstcapital.com Page 25#26LMM Portfolio Attributes Reflect Investment Strategy MAINST CAPITAL CORPORATION High yielding secured debt investments coupled with significant equity participation = Attractive risk-adjusted returns Weighted-Average Effective Yield=11.6% Average Fully Diluted Equity Ownership=41% Security Position on Debt Capital as a Percentage of Cost Fully Diluted Equity Ownership% 1st Lien 98% 2nd Lien/ Other 50.0% and greater 2% 28% 34% 38% 1.0% - 24.9% 25.0% - 49.9% Main Street Capital Corporation NYSE: MAIN mainstcapital.com Page 20#27Term and Total Interest Coupon of Existing LMM Debt Investments Original Term < 5 years 95% 4% 1% > 5 years 5 years Total Interest Coupon (1) N/A - Floating Interest Rate (Wtd. Avg. of 10.6%) MAINST CAPITAL CORPORATION 39% 11% 14% Current Interest <10% Current Interest 1% 7% 6% 10% Current Interest 11% Current Interest 34% 13% Current Interest 12% Current Interest Debt Investments generally have a 5-Year Original Term and -2.7 Year Weighted-Average Remaining Duration; Weighted-Average Effective Yield of 11.6% on Debt Portfolio (1) Interest coupon excludes amortization of deferred upfront fees, original issue discount, exit fees and any debt investments on non-accrual status (2) Floating interest rates generally include contractual minimum "floor" rates; Interest rate of 10.6% is based on weighted-average principal balance of floating rate debt investments as of June 30, 2020 Main Street Capital Corporation NYSE: MAIN mainstcapital.com Page 27#28Private Loan Investment Portfolio MAINST CAPITAL CORPORATION Private Loan Investment Portfolio provides a diversified mix of investments and sources of income to complement the LMM Investment Portfolio 64 investments/ $653.8 million in fair value 27% of total investment portfolio at fair value Average investment size of $11.7 million(1) (less than 1% of total portfolio) Investments in secured debt instruments • 92% of Private Loan portfolio is secured debt • 95% of Private Loan debt portfolio is first lien term debt Debt yielding 8.7% (2) 91% of Private Loan debt investments bear interest at floating rates (3), providing matching with MAIN's floating rate credit facility Approximately 575 basis point net cash interest margin vs "matched" floating rate on the MAIN credit facility (1) As of June 30, 2020; based on cost (2) Weighted-average effective yield includes amortization of deferred debt origination fees and accretion of original issue discount, but excludes fees payable upon repayment of the debt instruments and any debt investments on non-accrual status (3) 87% of floating interest rates on Private Loan debt investments are subject to contractual minimum "floor" rates Main Street Capital Corporation NYSE: MAIN mainstcapital.com Page 28#29Middle Market Investment Portfolio MAINST CAPITAL CORPORATION Middle Market Investment Portfolio provides a diversified mix of investments and diverse sources of income to complement the LMM Investment Portfolio and a potential source of liquidity for MAIN's future investment activities 44 investments/ $410.5 million in fair value 17% of total investment portfolio at fair value Average investment size of $11.7 million(1) (less than 1% of total portfolio) Investments in secured and/or rated debt investments • 94% of Middle Market portfolio is secured debt • 92% of Middle Market debt portfolio is first lien term debt Debt yielding 7.7% (2) 95% of Middle Market debt investments bear interest at floating rates(3), providing matching with MAIN's floating rate credit facility Approximately 500 basis point net cash interest margin vs "matched" floating rate on the MAIN credit facility More investment liquidity compared to LMM (1) As of June 30, 2020; based on cost (2) Weighted-average effective yield includes amortization of deferred debt origination fees and accretion of original issue discount, but excludes fees payable upon repayment of the debt instruments and any debt investments on non-accrual status (3) 74% of floating interest rates on Middle Market debt investments are subject to contractual minimum "floor" rates Main Street Capital Corporation NYSE: MAIN mainstcapital.com Page 29#30Private Loan & Middle Market Portfolios by Industry (as a Percentage of Cost) MAINST CAPITAL CORPORATION Aerospace & Defense, 9% Commercial Services & Supplies, 7% ■IT Services, 6% ■Machinery, 5% ■Internet Software & Services, 5% Hotels, Restaurants & Leisure, 4% ■Construction & Engineering, 4% ■Energy Equipment & Services, 2% ■Diversified Financial Services, 2% Food & Staples Retailing. 2% Transportation Infrastructure, 2% Internet & Catalog Retail, 2% ■Textiles, Apparel & Luxury Goods, 1% Health Care Providers & Services, 8% Communications Equipment, 6% ■Diversified Telecommunication Services, 5% ■ Oil, Gas & Consumable Fuels, 5% ■ Media, 5% ■Specialty Retail, 4% ■Leisure Equipment & Products, 3% ■Trading Companies & Distributors, 2% ■Distributors, 2% ■ Software, 2% ■Chemicals, 2% Professional Services, 1% ■ Other, 4% Main Street Capital Corporation NYSE: MAIN mainstcapital.com Page 30#31Diversified Private Loan & Middle Market Investments (as a Percentage of Cost) Invested Capital by Transaction Type Invested Capital by Geography (1) MAINST CAPITAL CORPORATION LBO/MBO 39% 40% Recapitalization/ Refinancing 2% 19% Growth Acquisition 21% 21% 22% 15% 21% (1) Based upon portfolio company headquarters and excluding any MAIN investments headquartered outside the U.S., which represent approximately 3% of the combined Private Loan and Middle Market portfolios Main Street Capital Corporation NYSE: MAIN mainstcapital.com Page 31#32Main Street Capital Corporation Financial Overview MAINST CAPITAL CORPORATION 2nd Quarter - 2020 Main Street Capital Corporation NYSE: MAIN mainstcapital.com Page 32#33MAIN Financial Performance Total Investment Income ($ in millions) Year over Year Growth MAINST CAPITAL CORPORATION Distributable Net Investment Income(3) ($ in millions) Year over Year Growth 8% 15% 13% 4% (12)%(1) 10% 17% 14% 1% (12)%(1) $260.0 $180.0 $240.0 $220.0 $243.4 $233.4 $160.0 $165.8 $167.4 $140.0 $200.0 $145.4 $205.7 $180.0 $120.0 $124.11 $178.3 $160.0 $164.6 $113.3 $100.0 $140.0 $120.0 $80.0 $100.0 $108.2 $73.5 $60.0 $80.0 $60.0 $40.0 $40.0 $20.0 $20.0 50.0 $0.0 2015 2016 2017 2018 2019 YTD (2) 2020 2015 2016 2017 2018 2019 YTD (2) 2020 (1) Reflects year-to-date June 30, 2020 performance compared with year-to-date June 30, 2019 performance (2) Through June 30, 2020 (3) See Non-GAAP Information included on page 47 of this presentation. Main Street Capital Corporation NYSE: MAIN mainstcapital.com Page 33