Margaret H. Warner - McDermott Will & Emery


Peg has served as first chair trial and appellate counsel in state and federal courts in over 30 jurisdictions. Her work has spanned antitrust, commercial, class action, environmental, healthcare, insurance, mass tort, product liability and reinsurance disputes. Peg has taken numerous cases to verdict/judgment before juries, judges and arbitration tribunals. Many of her cases have involved high-profile public policy issues from the Agent Orange litigation in the 1980s, environmental and Superfund litigation involving such notorious sites as Love Canal and the Hudson River in the 1990s, to sexual abuse, 9/11, antitrust, healthcare, ERISA and COVID-19 litigation in the 21st Century.

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  • Lead trial counsel for defendant fiduciaries in eight day ERISA class action bench trial. Obtained rulings denying all injunctive relief and 97% of damages sought. Full affirmance in Tenth Circuit
  • Lead trial counsel for defendant healthcare system defeating governmental effort to enjoin hospital closure; representation of healthcare systems in contractual and regulatory disputes nationwide
  • Lead trial counsel for plaintiff trade association in a two-week federal jury trial involving a nationwide retirement program for municipal workers. Verdict returned for client on all counts. Summary affirmance in DC Circuit

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  • Chambers USA, 2020 – 2022, Insurance: Insurer (Band 1)
  • Chambers USA, 2021 and 2022, Insurance: Dispute Resolution: Insurer
  • Legal 500 US, 2021 and 2022, Hall of Fame
  • The Best Lawyers in America, 2008 to 2023, Commercial Litigation, Insurance Law and Litigation – Environmental
  • Benchmark Litigation National, Litigation Star, Top 250 Women in LitigationShow More
  • Business Insurance 2008, named one of its 25 international Women to Watch
  • Chambers National Insurance Dispute Resolution – Insurer, District of Columbia Band 1
  • Super Lawyers

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University of Notre Dame, JD
St. Lawrence University, BA, cum laude

District of Columbia

US Supreme Court
US Court of Appeals for the Federal Circuit
US Court of Appeals for the District of Columbia Circuit
US Court of Appeals for the Second Circuit
US Court of Appeals for the Third Circuit
US Court of Appeals for the Fifth Circuit
US Court of Appeals for the Sixth Circuit
US Court of Appeals for the Seventh Circuit
US Court of Appeals for the Ninth Circuit
US Court of Appeals for the Tenth Circuit
District of Columbia Court of Appeals