WASHINGTON, D.C. (August 31, 2026) — Sullivan & Cromwell LLP announced today that Sonia Kuester Pfaffenroth has joined as a partner in the firm’s Antitrust Group, resident in its Washington, D.C. office. Ms. Pfaffenroth brings extensive experience advising clients on complex antitrust matters in the U.S. and globally, including having served as Deputy Assistant Attorney General for Civil and Criminal Operations, and Deputy Assistant Attorney General for Civil Enforcement in the Antitrust Division of the U.S. Department of Justice.
“S&C has long had a premier global antitrust practice. As antitrust enforcement and competition issues continue to shape business decisions around the globe, Sonia’s combination of government experience, technical excellence and strategic insight will enhance our ability to serve our clients’ increasingly complex needs,” said Sullivan & Cromwell Co-Chairs Robert Giuffra and Scott Miller. “We’re delighted to welcome her to S&C and look forward to working with her.”
Ms. Pfaffenroth’s practice encompasses federal and state antitrust litigation and investigations, as well as compliance, training and strategic counseling across a broad range of competition issues, including pricing, marketing, distribution, contracting, trade associations, R&D and other collaborations. She counsels on a broad range of antitrust matters including international cartel investigations, merger and acquisition reviews and civil antitrust litigation.
As Deputy Assistant Attorney General in the Antitrust Division, Ms. Pfaffenroth oversaw both civil and criminal antitrust enforcement efforts, as well as the Division’s work with antitrust and competition law enforcement agencies worldwide. She supervised a broad range of legal policy initiatives and competition advocacy efforts and oversaw a number of the Division’s most significant matters, including litigation challenging the Anthem/Cigna and Aetna/Humana mergers.
“Sonia’s deep understanding of antitrust issues and significant DOJ experience will be a tremendous asset to our clients and our team,” said Amanda Davidoff, Managing Partner of S&C’s Washington, D.C. office. “We’re thrilled to welcome her to our D.C. office.”
“I’m excited to join the outstanding S&C team whose work in the antitrust space is among the best in the world,” said Ms. Pfaffenroth. “I look forward to diving in and helping to address the wide variety of competition challenges S&C clients face.”
Ms. Pfaffenroth earned her J.D. from Stanford Law School, Order of the Coif, and her B.A. from Carleton College, magna cum laude. She clerked for Judge Paul Friedman of the U.S. District Court for the District of Columbia.
About Sullivan & Cromwell’s Antitrust Practice
Retained by some of the world’s largest companies on their most important matters, S&C’s award-winning Antitrust Group provides creative solutions to cross-border merger matters, criminal and civil antitrust investigations, and antitrust litigation. We provide a coordinated approach to securing merger clearances in the United States, the European Union and elsewhere. Our diversified client base includes companies in big tech, healthcare, financial services, sports and entertainment, manufacturing, oil and gas, consumer and retail, private equity, and aerospace and defense. S&C clients benefit from antitrust lawyers working hand-in-glove with global corporate and litigation teams, with our Washington, D.C., New York, California, London, and Brussels offices working especially closely on antitrust matters. The European team advises clients on the full range of EU, UK, German and French antitrust matters.
About Sullivan & Cromwell LLP
Sullivan & Cromwell LLP is a leading global law firm that advises on major domestic and cross-border M&A, significant litigation and corporate investigations, finance and corporate transactions, and complex antitrust, regulatory, tax and estate planning matters. Our firm’s hallmarks are the highest-quality independent advice and intense dedication to solving client problems. Founded in 1879, S&C has more than 1,000 lawyers located in offices in New York, Washington, D.C., Los Angeles, Palo Alto, London, Frankfurt, Paris, Brussels, Hong Kong, Beijing, Tokyo, Melbourne and Sydney.