Karen Kazmerzak is a Partner in the Firm’s Washington, D.C. office. A former lawyer within the Federal Trade Commission (FTC), Karen has extensive experience advising clients on antitrust concerns in connection with complex transactions, including obtaining merger clearance in the United States and developing strategies to advocate for antitrust approval in international jurisdictions.
Recognized as a leader in the field by Chambers USA (2018-2024), Legal 500 and a “Litigation Star” by Euromoney’s Benchmark Litigation (2023-2024), Karen is lauded by clients as “deeply knowledgeable, extremely practical and commercially minded.” She is also recognized for her work representing third-party market participants in connection with opposition to acquisitions and in connection with government subpoenas. Beyond her work for clients on merger clearance matters, she also advises on antitrust matters involving licensing, distribution, pricing and competitor collaborations.
Karen’s extensive private sector and government experience has spanned numerous sectors, including several regulated industries, such as life sciences, energy, transportation, healthcare and telecom, among many others.
Prior to joining Simpson Thacher, Karen was co-leader of the global Antitrust/Competition practice at another international firm. She served as an attorney with the FTC from 2006 to 2010.
Karen is an established thought leader on antitrust topics and is an active member of the American Bar Association’s Antitrust Section. She currently serves as an associate editor of Antitrust and previously served as a vice chair of the Transportation and Energy Industries Committee (2012–2018). Karen also co-founded the Women’s Antitrust Forum, a Washington, D.C.-based organization that is focused on bringing together and creating development opportunities for women practicing antitrust law, policy and economics.
Updated Oct 2025
Amedisys in favorable settlement of DOJ litigation to block its $3.3 billion acquisition by UnitedHealth Group.
Beacon Roofing Supply in connection with its acquisition by QXO.
Herc Rentals in its $5.3 billion acquisition of H&E Equipment Services, following H&E’s termination of its previously announced proposed sale to United Rentals.
Advanced Instruments in connection with its $2.2 billion acquisition of and merger with Nova Biomedical.
- Radius Recycling in connection with its announced acquisition by Toyota Tsusho America for $1.34 billion.
- Competition/antitrust
- Consumer goods and services
- Energy
- Financial services
- Healthcare
- Oil and gas
- Pharmaceutical and life sciences
- Technology and telecommunications
- Transport
- J.D. - University of Iowa College of Law - 2006
- B.S., Microbiology - University of Iowa - 1998
- American Bar Association Section of Antitrust
- Women's Antitrust Forum
- District of Columbia - 2011
- New York - 2007