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Rachel Mossman Zieminski


Rachel is an experienced litigator and trial attorney representing clients in some of the most complex and significant antitrust disputes in the market today. These include cases challenging clients’ intellectual property under antitrust theories, defending clients’ hiring practices against “no-poach” challenges, and defending mergers in trials against the Department of Justice.

Rachel has over a decade of experience in antitrust cases involving healthcare and represents clients across industries including health insurance, retail, payment processing, aviation, book publishing, defense contracting, and private equity.

Rachel has a passion for fact gathering and a unique ability to distill complicated antitrust arguments into easy-to-understand narratives. These skills allow Rachel to develop strong discovery records and compelling arguments in support of her clients’ positions.


Representative matters

Health insurer in a multidistrict litigation and related opt-out litigations alleging that license agreements, membership standards, and guidelines entered into by insurer violate federal and state antitrust laws.

Contracting firm in a class action antitrust litigation challenging the hiring practices of defense contractors at a defense intelligence operations center located in England.

Airline in trial against DOJ challenging merger agreement under Section 7 of the Clayton Act.

Book publisher in trial against DOJ challenging merger agreement under Section 7 of the Clayton Act. 

Pro bono

Representing individuals and community groups in suit and trial challenging Arkansas’ long-standing method of electing Court of Appeals Judges and Supreme Court Justices under Section II of the Voting Rights Act, because it denies Black Arkansans an equal opportunity to elect candidates of their choice.

Training to wildlife officers on preserves in Tanzania on how to collect evidence and testify at trial in poaching prosecution cases.

Published Work

  • Mossman Zieminski, Rachel, Co-author. (2019-2023) “USA: Competition Litigation”, International Comparative Legal Guide – Competition Litigation
  • Mossman, Rachel. (2015) “States Writing Biometric-Capture Laws May Look To Illinois”, Law360

Speaking Engagements

  • Speaker, “The Pros and Cons of Non-Competes: The Economic Debate,” Competition Law Institute’s 50th Annual Conference on International Antitrust Law and Policy, September 2023
  • Speaker, “Antitrust Trends: M&A and Employer-Employee Relations,” Michigan Association of Corporate Counsel, April 2023
  • Speaker, “Why Antitrust? For Lawyers,” American Bar Association, July 2020


  • Selected to the “40 & Under List”, Benchmark Litigation, 2022 & 2023
  • The Legal 500 in Antitrust: Civil/Class Action Litigation



Texas, 2022
District of Columbia, 2013
Virginia, 2012


Sixth Circuit Court of Appeals, 2019
United States District Court for the District of Columbia, 2018
United States District Court for the Eastern District of Michigan, 2016


B.A., Classics, History, College of Charleston Honors College, summa cum laude, 2009
J.D., University of Virginia School of Law, 2012
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.