Filed by MDC Partners Inc.

pursuant to Rule 425 under the U.S. Securities Act of 1933, as amended,

and deemed filed pursuant to Rule 14a-12

under the Securities Exchange Act of 1934, as amended

Subject Company: MDC Partners, Inc.

Commission File No.: 001-13718

Date: December 29, 2020

In connection with MDC Partners Inc.’s (“MDC”) proposed business combination with Stagwell Media LP, Mark Penn, Chairman and CEO of MDC and Managing Partner of The Stagwell Group, was interviewed by Maria Baritromo of Fox Business on December 24, 2020. A transcript of the interview follows below.

MDC Partners CEO on Stagwell merger

Maria Bartiromo: Good morning. Major marketing merger: MDC Partners and Stagwell announcing that the two companies will combine their businesses to form a massive global marketing network. The company is targeting growth to over 3 billion dollars in revenue by 2025. Joining me now, former Senior Advisor to the Clintons and also MDC Partners CEO and Stagwell Group president, Mark Penn. Mark, good morning, great to see you.

Mark Penn: Good morning.

MB: Let’s talk about the merger. Let’s talk about the kind of growth that you’re expecting and how specifically you’re using technology to increase the footprint and get that marketing message out there.

MP: Sure, this merger brings together incredible creative talent along with digital and technology talent that works for the modern consumer and the modern marketer. We just heard about the incredible explosion of online shopping and experience and this is really the first scaled creative performance marketing firm designed with the right technology and talent in order to hit that consumer. That’s what’s going to give us tremendous growth in this market place.

MB: So let’s talk about that a little bit because it’s so interesting, especially with the backdrop of the pandemic right now, so many businesses have been struggling but in some ways, and not to diminish the human tragedy here, but in some ways, your business can actually profit from the shifts that we’re seeing here.

MP: Well, it’s been a tough pandemic for all of us, but I think that marketing is playing an essential role here. And when you look at the kind of company we have, you think of car companies. Is Tesla different from those companies making other cars? Yes. We’re kind of the Tesla of marketing and advertising, because we’re really built for the digital consumer, we’re built for this new online world, we’re built to get the right ad to the right person at the right time. It’s a whole new marketing and we are right in the center of it in 23 different countries and over 8,000 people.

MB: Yeah, that’s a huge workforce and its key to bring these companies together at this time. Do you anticipate, especially as we’re waiting for vaccine rollouts and for things to slowly get back to normal as we’re still in the thick of fighting this second wave, do you expect challenges in 2021?

MP: I do think that there are challenges here while the vaccines roll in, but I do think by next summer there’s going to be an explosion here of travel and pent up, even revenge consumerism, that is really going to propel the economy, and as the economy gets propelled, so does marketing, and if it’s marketing, it also is Stagwell and MDC. Which, by the way, MDC Partners is on the NASDAQ. We hope that this will really be a win-win for the world, for the country, and for consumers.

Cautionary Statement Regarding Forward-Looking Statements

This communication may contain certain forward-looking statements (collectively, “forward-looking statements”) within the meaning of Section 27A of the U.S. Securities Act of 1933, as amended and Section 21E of the U.S. Exchange Act and the United States Private Securities Litigation Reform Act of 1995, as amended, and “forward-looking information” under applicable Canadian securities laws. Statements in this document that are not historical facts, including statements about MDC’s or Stagwell’s beliefs and expectations and recent business and economic trends, constitute forward-looking statements. Words such as “estimate,” “project,” “target,” “predict,” “believe,” “expect,” “anticipate,” “potential,” “create,” “intend,” “could,” “should,” “would,” “may,” “foresee,” “plan,” “will,” “guidance,” “look,” “outlook,” “future,” “assume,” “forecast,” “focus,” “continue,” or the negative of such terms or other variations thereof and terms of similar substance used in connection with any discussion of current plans, estimates and projections are subject to change based on a number of factors, including those outlined in this section. Such forward-looking statements may include, but are not limited to, statements related to: future financial performance and the future prospects of the respective businesses and operations of MDC, Stagwell and the combined company; information concerning the proposed business combination with subsidiaries of Stagwell (the “Proposed Transaction”); the anticipated benefits of the Proposed Transaction; the likelihood of the Proposed Transaction being completed; the anticipated outcome of the Proposed Transaction; the tax impact of the Proposed Transaction on MDC and shareholders of MDC; the timing of the shareholder meeting to approve the Proposed Transaction (the “Special Meeting”) the shareholder approvals required for the Proposed Transaction; regulatory and stock exchange approval of the Proposed Transaction; and the timing of the implementation of the Proposed Transaction. A number of important factors could cause actual results to differ materially from those contained in any forward-looking statement, including the risks identified in our filings with the SEC.

These forward-looking statements are subject to various risks and uncertainties, many of which are outside MDC’s control. Important factors that could cause actual results and expectations to differ materially from those indicated by such forward-looking statements include, without limitation, the risks and uncertainties set forth under the section entitled “Risk Factors” in the Proxy/Prospectus, and under the caption “Risk Factors” in MDC’s Annual Report on Form 10-K for the year-ended December 31, 2019 under Item 1A, in MDC’s Quarterly Report on Form 10-Q for the three-months ended March 31, 2020 under Item 1A, in MDC’s Quarterly Report on Form 10-Q for the six-months ended June 30, 2020 under Item 1A and in MDC’s Quarterly Report on Form 10-Q for the nine-months ended September 30, 2020. These and other risk factors include, but are not limited to, the following:

You can obtain copies of MDC’s filings under its profile on SEDAR at www.sedar.com, its profile on the SEC’s website at www.sec.gov or its website at www.mdc-partners.com. MDC does not undertake any obligation to update any forward-looking statements as a result of new information, future developments or otherwise, except as expressly required by law. All forward-looking statements in this communication are qualified in their entirety by this cautionary statement.

Additional Information and Where to Find It

In connection with the Proposed Transaction, MDC and New MDC will file with the SEC a registration statement on Form S-4 (the “Form S-4”) that will include a proxy statement of MDC (the “Proxy Statement” and, together with the Form S-4, the “Proxy Statement/Prospectus”). This communication is not a substitute for the Proxy Statement/Prospectus or any other document MDC may file with the SEC in connection with the Proposed Transaction. When available, MDC will mail the Proxy Statement/Prospectus to its shareholders in connection with the votes to approve certain matters in connection with the Proposed Transaction.

INVESTORS AND SECURITYHOLDERS OF MDC ARE URGED TO READ CAREFULLY THE PROXY STATEMENT/PROSPECTUS, ONCE AVAILABLE, REGARDING THE PROPOSED TRANSACTION IN ITS/THEIR ENTIRETY WHEN THEY BECOME AVAILABLE (INCLUDING ANY AMENDMENTS OR SUPPLEMENTS THERETO) OR ANY DOCUMENTS WHICH ARE INCORPORATED BY REFERENCE IN THE PROXY STATEMENT/PROSPECTUS, BECAUSE THEY WILL CONTAIN IMPORTANT INFORMATION ABOUT THE PROPOSED TRANSACTION. You may obtain, free of charge, copies of the Proxy Statement/Prospectus, when available, and other relevant documents filed by MDC or New MDC with the SEC, at the SEC’s website at www.sec.gov. In addition, investors and securityholders will be able to obtain free copies of the Proxy Statement/Prospectus and other relevant documents filed by MDC or New MDC with the SEC and from MDC’s website at http://www.mdc-partners.com.

The URLs in this announcement are intended to be inactive textual references only. They are not intended to be active hyperlinks to websites. The information on such websites, even if it might be accessible through a hyperlink resulting from the URLs or referenced herein, is not and shall not be deemed to be incorporated into this announcement. No assurance or representation is given as to the suitability or reliability for any purpose whatsoever of any information on such websites.

No Offer or Solicitation

This communication does not constitute an offer to buy or exchange, or the solicitation of an offer to sell or exchange, any securities, nor shall there be any sale of securities in any jurisdiction in which such offer, sale or exchange would be unlawful prior to registration or qualification under the securities laws of any such jurisdiction. This communication is not a substitute for any prospectus, proxy statement or any other document that MDC or New MDC may file with the SEC in connection with the Proposed Transaction. No money, securities or other consideration is being solicited, and, if sent in response to the information contained herein, will not be accepted.

No offering of securities shall be made except by means of a prospectus meeting the requirements of the U.S. Securities Act of 1933, as amended. The Proposed Transaction and distribution of this document may be restricted by law in certain jurisdictions and persons into whose possession any document or other information referred to herein should inform themselves about and observe any such restrictions. Any failure to comply with these restrictions may constitute a violation of the securities laws of any such jurisdiction. No offering of securities will be made directly or indirectly, in or into any jurisdiction where to do so would be inconsistent with the laws of such jurisdiction.

Participants in the Solicitation

MDC, New MDC and their respective directors and executive officers and other members of management and employees, may be deemed to be participants in the solicitation of proxies from MDC’s shareholders with respect to the approvals required to complete the Proposed Transaction. More detailed information regarding the identity of these potential participants, and any direct or indirect interests they may have in the Proposed Transaction, by security holdings or otherwise, will be set forth in the Proxy Statement/Prospectus when filed with the SEC. Information regarding MDC’s directors and executive officers is set forth in the definitive proxy statement on Schedule 14A filed by MDC with the SEC on May 26, 2020 and in the Annual Report on Form 10-K filed by MDC with the SEC on March 5, 2020. Additional information regarding the interests of participants in the solicitation of proxies in respect of the Special Meeting will be included in the Proxy Statement/Prospectus to be filed with the SEC. These documents are available to the shareholders of MDC free of charge from the SEC’s website at www.sec.gov and from MDC’s website at www.mdc-partners.com.

You must not construe the contents of this document as legal, tax, regulatory, financial, accounting or other advice, and you are urged to consult with your own advisors with respect to legal, tax, regulatory, financial, accounting and other consequences of the Proposed Transaction, the suitability of the Proposed Transaction for you and other relevant matters concerning the Proposed Transaction.