Exhibit 99.1

	NEWS RELEASE
	Contacts: Main Street Capital Corporation Dwayne L. Hyzak, CEO, dhyzak@mainstcapital.com Jesse E. Morris, CFO and COO, jmorris@mainstcapital.com 713-350-6000 Dennard Lascar Investor Relations Ken Dennard / ken@dennardlascar.com Zach Vaughan / zvaughan@dennardlascar.com 713-529-6600

Main Street Announces Preliminary Estimate of First Quarter 2023 Operating Results

Announces First Quarter 2023 Earnings Release and Conference Call Schedule

HOUSTON – April 18, 2023 – Main Street Capital Corporation (NYSE: MAIN) (“Main Street” or the “Company”) is pleased to announce its preliminary operating results for the first quarter of 2023 and its first quarter 2023 earnings release and conference call schedule.

In commenting on the Company’s operating results for the first quarter of 2023, Dwayne L. Hyzak, Main Street’s Chief Executive Officer, stated, “We are very pleased with our performance in the first quarter, which resulted in new quarterly records for net investment income per share, distributable net investment income per share and net asset value per share for the third consecutive quarter and a return on equity of over 14%. Our performance included positive results from our lower middle market and private loan investment strategies and significant contributions from our asset management business, continuing our positive momentum from the last few quarters. These results demonstrate the continued and sustainable strength of our overall platform, the benefits of our differentiated and diversified investment strategies, the unique contributions of our asset management business and the underlying quality of our portfolio companies. We are also pleased that we ended the first quarter with an attractive pipeline of investment opportunities in our lower middle market and private loan investment strategies, which we expect will be helpful as we work to maintain our positive momentum from the last few quarters.”

Mr. Hyzak continued, “Our distributable net investment income in the first quarter exceeded the monthly dividends paid to our shareholders by over 55% and the total dividends paid to our shareholders by over 20%, allowing us to deliver significant value to our shareholders while still conservatively retaining a meaningful portion of our income. Our continued positive performance over the last few quarters has allowed us to deliver significantly increased value to our shareholders through the two sequential quarterly increases to our regular monthly dividends in the fourth quarter of 2022 and the first quarter of 2023 and the consistent payment of meaningful quarterly supplemental dividends. In addition, based on our strong first quarter results, we expect another meaningful supplemental dividend to be paid in the second quarter of 2023, which would represent our seventh consecutive quarterly supplemental dividend and would result in another quarter in which we provide significant additional value to our shareholders.”

Preliminary Estimates of First Quarter 2023 Results

Main Street’s preliminary estimates of first quarter 2023 net investment income (“NII”) is $1.01 to $1.03 per share and distributable net investment income (“DNII”), which is NII before non-cash compensation expenses, is $1.06 to $1.08 per share.(1)

Main Street’s preliminary estimate of net asset value (“NAV”) per share as of March 31, 2023 is $27.20 to $27.26, representing an increase of $0.34 to $0.40 per share, or 1.3% to 1.5%, from the NAV per share of $26.86 as of December 31, 2022, with this increase after the impact of the supplemental dividend paid in March 2023 of $0.175 per share. The estimated increase in NAV per share is primarily due to the record estimated NII, which exceeded the regular monthly dividends of $0.675 per share and supplemental dividend of $0.175 per share paid in the first quarter, the accretive impact from equity issuances during the quarter and the net fair value increase on the investment portfolio resulting from the net unrealized appreciation on our portfolio investments, partially offset by net realized losses on our portfolio investments and the tax provision resulting from the net realized and unrealized fair value changes on our investment portfolio.

As a result of Main Street’s preliminary estimates of net investment income and net changes in the fair market value of its investment portfolio as noted above, Main Street estimates that it generated a quarterly annualized return on equity for the first quarter of 2023 of 14% to 15%.(2)

Investment Portfolio Activity

The Company’s first quarter 2023 operating activities include the following investment activity in our LMM, private loan and middle market investment strategies:

•$58.7 million in total LMM portfolio investments, which after aggregate repayments of debt principal from several LMM portfolio investments and a decrease in cost basis due to the realized loss on a LMM portfolio investment resulted in a net increase of $7.6 million in the cost basis of our LMM investment portfolio;

•$43.7 million in total private loan investments, which after aggregate repayments resulted in a net increase of $23.6 million in the cost basis of our private loan investment portfolio; and

•a net decrease of $12.2 million in the cost basis of our middle market investment portfolio.

First Quarter 2023 Earnings Release and Conference Call Scheduled

Main Street’s first quarter 2023 results will be released on Thursday, May 4, 2023, after the financial markets close. In conjunction with the release, Main Street has scheduled a conference call, which will be broadcast live via phone and over the Internet, on Friday, May 5, 2023, at 10:00 a.m. Eastern time. Investors may participate either by phone or audio webcast.(3)

By Phone:    Dial 412-902-0030 at least 10 minutes before the call. A replay will be available through May 12, 2023 by dialing 201-612-7415 and using the access code 13738037#.

By Webcast:    Connect to the webcast via the Investor Relations section of Main Street’s website at www.mainstcapital.com. Please log in at least 10 minutes in advance to register

and download any necessary software. A replay of the conference call will be available on Main Street’s website shortly after the call and will be accessible for approximately 90 days.

ABOUT MAIN STREET CAPITAL CORPORATION

Main Street (www.mainstcapital.com) is a principal investment firm that primarily provides long-term debt and equity capital to lower middle market companies and debt capital to middle market companies. Main Street’s portfolio investments are typically made to support management buyouts, recapitalizations, growth financings, refinancings and acquisitions of companies that operate in diverse industry sectors. Main Street seeks to partner with entrepreneurs, business owners and management teams and generally provides “one stop” financing alternatives within its lower middle market investment strategy. Main Street’s lower middle market companies generally have annual revenues between $10 million and $150 million. Main Street’s middle market debt investments are made in businesses that are generally larger in size than its lower middle market portfolio companies.

Main Street, through its wholly owned portfolio company MSC Adviser I, LLC (“MSC Adviser”), also maintains an asset management business through which it manages investments for external parties. MSC Adviser is registered as an investment adviser under the Investment Advisers Act of 1940, as amended.

FORWARD-LOOKING STATEMENTS AND OTHER MATTERS

Main Street cautions that statements in this press release which are forward-looking and provide other than historical information, including but not limited to the preliminary estimates of first quarter 2023 financial information and results and the declaration and payment of future dividends, are based on current conditions and information available to Main Street as of the date hereof. Although its management believes that the expectations reflected in those forward-looking statements are reasonable, Main Street can give no assurance that those expectations will prove to be correct. Those forward-looking statements are made based on various underlying assumptions and are subject to numerous uncertainties and risks, including, without limitation, such factors described under the captions “Cautionary Statement Concerning Forward-Looking Statements” and “Risk Factors” included in Main Street’s filings with the Securities and Exchange Commission (the “SEC”) (www.sec.gov). Main Street undertakes no obligation to update the information contained herein to reflect subsequently occurring events or circumstances, except as required by applicable securities laws and regulations.

The preliminary estimates of first quarter 2023 financial information and results furnished above are based on Main Street management’s preliminary determinations and current expectations, and such information is inherently uncertain. The preliminary estimates are subject to completion of Main Street’s customary quarter-end closing and review procedures and third-party review, including the determination of the fair value of Main Street’s portfolio investments. As a result, actual results could differ materially from the current preliminary estimates based on adjustments made during Main Street’s quarter-end closing and review procedures and third-party review, and Main Street’s reported information in its Quarterly Report on Form 10-Q for the quarter ended March 31, 2023 may differ from this information, and any such differences may be material. In addition, the information furnished above does not include all of the information regarding Main Street’s financial condition and results of operations for the quarter ended March 31, 2023 that may be important to readers. As a

result, readers are cautioned not to place undue reliance on the information furnished in this press release and should view this information in the context of Main Street’s full first quarter 2023 results when such results are disclosed by Main Street in its Quarterly Report on Form 10-Q for the quarter ended March 31, 2023. The information furnished in this press release is based on current Main Street management expectations that involve substantial risks and uncertainties that could cause actual results to differ materially from the results expressed in, or implied by, such information.

Main Street has an existing effective shelf registration statement on Form N-2 on file with the SEC relating to the offer and sale from time to time of its securities. Investors are advised to carefully consider the investment objective, risks and charges and expenses of Main Street before investing in any of Main Street’s securities. The prospectus included in the shelf registration statement, together with any related prospectus supplement, contain this and other information about Main Street and should be read carefully before investing. A copy of the prospectus and any related prospectus supplement may be obtained by contacting Main Street.

End Notes

(1)     Distributable net investment income is net investment income as determined in accordance with U.S. Generally Accepted Accounting Principles, or U.S. GAAP, excluding the impact of non-cash compensation expenses, which includes both share-based compensation expenses and deferred compensation expense or benefit. Main Street believes presenting distributable net investment income per share is useful and appropriate supplemental disclosure for analyzing its financial performance, since share-based compensation does not require settlement in cash and deferred compensation expense or benefit does not result in a net cash impact to Main Street upon settlement. However, distributable net investment income is a non-U.S. GAAP measure and should not be considered as a replacement for net investment income and other earnings measures presented in accordance with U.S. GAAP. Instead, distributable net investment income should be reviewed only in connection with such U.S. GAAP measures in analyzing Main Street’s financial performance. In order to reconcile estimated distributable net investment income per share to estimated net investment income per share in accordance with U.S. GAAP for the first quarter of 2023, $0.05 to $0.06 per share of estimated non-cash compensation expenses are added back to calculate estimated distributable net investment income per share.

(2)    Return on equity equals the net increase in net assets resulting from operations divided by the average quarterly total net assets.

(3)     No information contained on our website or disclosed on the May 5, 2023 conference call, including the webcast and the archived versions, is incorporated by reference in this press release or any of our filings with the SEC, and you should not consider that information to be part of this press release or any other such filing.