Catherine Simonsen
Partner
Los Angeles, CA
T +1 917-747-5196
E catherine@simonsensussman.com
Overview
Catherine is a Partner and Co-Founder of the firm. She brings over a decade of experience investigating and litigating government and private antitrust cases on both the plaintiff and defense sides in federal and California state courts.
Prior to co-founding the firm, Catherine served as the Assistant Regional Director for the Western Competition Group at the Federal Trade Commission, where she managed merger and conduct investigations and complaint preparation for violations of Clayton Act Section 7, the Robinson-Patman Act, Sherman Act Sections 1 and 2, and the Federal Trade Commission Act. Before joining the FTC, Catherine served as a Deputy Attorney General in the Antitrust Section of the California Attorney General’s office, where she investigated and litigated the State’s California Cartwright Act and Unfair Competition Law claims against Amazon.
Prior to her government enforcement roles, Catherine was a partner in the litigation group of an Am Law 100 law firm. She successfully prosecuted numerous competitor suits under federal and state antitrust laws, including defeating several motions to dismiss, summary judgment motions, and Daubert motions and taking an over-$100-million antitrust damages case to trial in federal court resulting in a favorable settlement for her client.
Catherine's appellate experience includes successes before state and federal appeals courts. In the U.S. Court of Appeals for the Ninth Circuit, she secured affirmance of her client’s attorney’s fees award. In the U.S. Court of Appeals for the Third Circuit, Catherine secured affirmance of a class certification decision in a large, multi-state antitrust class action. In the U.S. Court of Appeals for the Eighth Circuit, she secured reversal of the district court’s order remanding a putative antitrust class action against her client to state court. In addition, she has represented clients appearing as amici curiae in the Washington Supreme Court and U.S. Supreme Court and at the certiorari and merits stages at the U.S. Supreme Court.
Catherine is dedicated to public interest and pro bono work. She currently serves as Senior Legal Fellow with the American Economic Liberties Project, where she advises on antitrust reform legislation and has prepared several practice guides for high-impact enforcement of the antitrust laws and co-authored an amicus brief on monopolization remedies. In her pro bono work, Catherine has represented Salvadoran nationals seeking asylum, as well as a variety of non-profits before the U.S. Supreme Court, including women's rights and gender justice organizations and a grassroots political organization. For over a decade before entering government service, Catherine volunteered with and served in several leadership roles with the non-profit mentoring organization Minds Matter, including founding the Seattle chapter in 2013.
Catherine clerked for the Honorable J. Jerome Farris of the U.S. Court of Appeals for the Ninth Circuit and the Honorable John C. Coughenour of the U.S. District Court for the Western District of Washington. She holds a J.D. from Yale Law School and a B.A. from Stanford University.
Prior to her legal career, Catherine was an associate in the equities division of a global investment bank in New York.
Selected Speaking Engagements
Panelist, Monopolization and Litigation, Antitrust West Coast, May 2025
Panelist and Honoree, Celebrating Women in Competition Law in California, California Lawyers Association, March 2025
Panelist, Institute for Corporate Counsel, Los Angeles County Bar Association, December 2024
Panelist, Trends in Antitrust Enforcement, Los Angeles Chapter of the Federal Bar Association, April 2024
Bars and Courts
California State Bar
Washington State Bar
U.S. Court of Appeals for the Ninth Circuit
U.S. Court of Appeals for the Third Circuit
U.S. District Court for the Central District of California
U.S. District Court for the Eastern District of California
U.S. District Court for the Northern District of California
U.S. District Court for the Southern District of California
U.S. District Court for the Western District of Washington