Mallory Brennan

Mallory Tosch Brennan


Mallory focuses on representing global corporations and financial institutions in mergers and acquisitions litigation and transactional disputes, securities litigation and other complex commercial disputes, including bankruptcy and antitrust actions. She also has experience counseling multinational corporations in connection with both internal and regulatory investigations.
Mallory represents corporates in M&A transactions on both the buy-side and sell-side, as well as financial institutions advising on M&A transactions, including in shareholder class actions, busted deals, post-transaction disputes, and regulatory inquiries. She is experienced in conducting internal investigations into corporate governance irregularities, insider trading, FCPA violations, and other regulatory matters.


Representative matters

M&A Litigation

Pattern Energy Group in litigation arising out of its acquisition by CPPIB.

Intercontinental Exchange Inc. (ICE) in litigation arising out of its acquisition of Black Knight.

Microvast Holdings Inc. in litigation arising out of its deSPAC transaction with Tuscan.

A large investment bank in connection with a Special Litigation Committee investigation into defense advisory services to a client facing an activist investor campaign.

A large investment bank in connection with the Alta Mesa deSPAC.

A large private equity oil & gas investor in midstream assets in various disputes, including with LLC co-owners and with a joint venture purchaser.

A midstream oil & gas company in a post-closing earnout dispute.

Liberty Global in connection with an action relating to the acquisition of Sunrise. 

A large investment bank in connection with litigation challenging a founder’s interests in a subsidiary.

Raytheon in the United Technologies merger.

Goldman Sachs in litigation arising out of the sale of Saks Fifth Avenue.

LyondellBasell in the A. Schulman acquisition.

Liberty Global in connection with litigation arising out of the Liberty Broadband acquisition of Charter Communications.

Capital Square Partners in the Aegis/Startek acquisition.

CVS in the Aetna acquisition.

Altice in the Cablevision acquisition.

Dassault in the Exa acquisition.

WebMD Health Corporation in its acquisition by Internet Brands, a portfolio company of Kohlberg Kravis Roberts & Co. L.P. (KKR).

Pall in its acquisition by Danaher.

Daimler AG in connection with various Chrysler-related post-demerger litigations and disputes.Huntsman Corp. in a merger dispute with Hexion Chemicals Corp. 

Merrill Lynch in connection with shareholder litigation arising out of its merger with Bank of America.

BCE Inc. against former subsidiary of Teleglobe Inc.

Securities-Related Litigation

A large telecom company in multiple litigations challenging various disclosures made in securities filings.

Altice USA and related entities and individuals in securities litigation arising out of IPO and in connection with Section 16(b) claims.

Jefferies in a securities litigation related to the acquisition by Landcadia Holdings, Inc., a special purpose acquisition company (“SPAC”), of Waitr Holdings, Inc. 
Orbital ATK in securities litigation arising out of financial restatement.

Underwriting syndicates, including clients such as Bank of America, Citigroup, Merrill Lynch, Credit Suisse and dozens of others, in separate securities class actions arising out of recent stock offerings of companies such as ZTO Express and Arconic.

Countrywide and related entities in ongoing institutional RMBS purchaser actions filed in state and federal courts across the United States by institutional plaintiffs such as federal home loan banks, mutual and hedge funds, insurance companies and regional banks.

Barclays in the FHLBS RMBS litigation.

Countrywide with respect to litigation concerning CDOs.

Todd Newman, former portfolio manager at Diamondback Capital, in connection with the reversal of his insider trading conviction.

White-Collar and Regulatory Investigations

A major international financial institution in an internal investigation into mutual fund investment irregularities at a Mexican subsidiary, including potential insider trading and related internal governance issues.

An international engineering and construction company in a criminal investigation by the United States Department of Justice for possible FCPA violations.

Former executives of the South American subsidiary of a multinational company in connection with an FCPA investigation and a potential criminal investigation by local authorities.

Employee of foreign automotive manufacturer in connection with investigation by U.S. authorities into potential regulatory, criminal, and antitrust violations.

Antitrust Litigation

A defense intelligence contractor in a no-poach antitrust class action.

A financial institution in the Mexican Government Bonds antitrust class action.

A financial institution in the ISDAFIX antitrust class action.

A financial institution in a class action asserting violations of the Commodity Exchange Act and antitrust laws related to Yen LIBOR and TIBOR.

A major European cargo airline in a criminal price-fixing cartel investigation and the follow-on federal class action lawsuit and related opt-out actions alleging a conspiracy to fix the prices of air cargo.

Additional Complex Commercial Experience

A startup hydrogen company in a breach of contract litigation with a services provider.

Citigroup in a class action concerning purported misconduct in Mexico.
Shorenstein Properties in connection with a commercial property dispute.

Deutsche Bank in a foreclosure litigation in connection with a distressed hotel property.

Published Work

  • Co-author, (2024) “6 Focus Areas For Companies Managing The Risks Of AI Use”, Law360
  • Co-author, (2023) “Delaware Court of Chancery Grants Plaintiff Attorney’s Fees Award Under Corporate Benefit Doctrine for Demand to SPAC Board Leading to Adjusted Voting Structure in Connection with Merger”, The M&A Lawyer
  • Author, (2022) “Delaware Court of Chancery Finds Personal Jurisdiction Over LLC ‘Acting Manager’ in Post-Closing Investor Action Challenging Merger with SPAC”, The M&A Lawyer
  • Author, (2022) “Delaware Supreme Court Reverses Dismissal of a Post-Merger Suit for Alleged Breach of Fiduciary Duty Related to Disclosures on Appraisal Rights”, The M&A Lawyer

Speaking Engagements

  • Co-Presenter, The Evolving Landscape of ESG Litigation, University of Texas School of Law/The Review of Litigation Symposium: The ESG Legal Landscape, February 2024
  • Co-presenter, Social Media and AI – Ethical Guidelines for Lawyers, Texas General Counsel Forum, May 2023
  • Panelist, Taking & Defending Powerful Depositions, The Rossdale Group, July 2020

Leadership Positions And Professional Affiliations

  • Board of Directors, French American Chamber of Commerce – Texas
  • Board of Directors, American Association for the International Commission of Jurists
  • Member, New York State Bar Association


  • Up & Coming in Securities Litigation – Texas, Chambers USA, 2022-2023
  • Next Generation Lawyer for M&A Litigation Defense, The Legal 500, 2017-2023
  • Top Ten Most Influential Securities Litigation Lawyers in Texas, Business Today, 2023
  • 40 & Under Hot List, Benchmark Litigation, 2018-2020
  • Rising Star, New York Law Journal 
  • Rising Star, Euromoney Legal Media Group



Texas, 2021
New York, 2008


United States District Courts for the Southern and Eastern Districts of New York
United States District Court for the Eastern District of Michigan
United States Court of Appeals for the Fifth Circuit


B.S., Millsaps College, 2002
J.D., Seton Hall University, 2007


A&O Shearman was formed on May 1, 2024 by the combination of Shearman & Sterling LLP and Allen & Overy LLP and their respective affiliates (the legacy firms). Any matters referred to above may include matters undertaken by one or more of the legacy firms rather than A&O Shearman.