John S. Kiernan is a retired partner and currently is of counsel to the firm. Mr. Kiernan served on the firm’s Management Committee and as Co-Chair of its Litigation Department for thirteen years, and was Chair of its Ethics Committee from 1994-2021. His representations have embraced a broad range of commercial disputes and internal investigations, including disputes relating to contracts, purchases and sales of businesses, corporate governance, derivative and class action claims, international treaties, securities claims, patents and other intellectual property, consumer fraud, accountant liability and mass torts.

Mr. Kiernan is recognized as a leading litigator by numerous publications. According to Chambers USA, he is “extremely personable, superior in intellect and yet extremely practical” (2022) and “an extraordinary commercial litigator” whose “instincts and analysis are always spot-on” (2021). Clients praise his “great judgment” (2010) and recognize him as “technically excellent” (2009), “incredibly creative,” “great when put in front of a board of directors” (2012) and a “superb litigator” (2011), with peers describing him as “an extremely bright, first-class litigator, thinker and colleague” (2008) and a “superstar” who applies a “deft touch rather than a sledgehammer” in commercially sensitive contract disputes, international treaty claims and securities actions (2007). Clients say he is “very balanced, and has real gravitas. He’s as cool as can be in incredibly intense situations” (2020). In addition, IFLR Benchmark Litigation Guide has ranked Mr. Kiernan among the leading commercial litigators in the United States and in New York each year since 2010 and notes “his extensive experience and breadth of knowledge.” He is also recognized as a leading securities litigator in The Legal 500 US (2017) and as a leading litigator for commercial litigation or alternative methods of dispute resolution in numerous other “Best Lawyers” publications.

Mr. Kiernan is the Board Chair of Prisoners’ Legal Services of New York and Co-Chair of the Inner City Scholarship Fund, Lawyers Division. He has previously chaired the Boards of the International Institute for Conflict Prevention & Resolution (CPR), Legal Services-New York City, the Lawyers’ Committee for Civil Rights Under Law, Volunteers of Legal Service, the Justice Resource Center and the New York City Bankruptcy Assistance Project (which he co-founded), and he was the Mayor of Pelham Manor, NY from 1999 to 2001. He has also served as a director or trustee of the City Bar Justice Center, New York Alliance for the Public Schools, Practicing Attorneys for Law Students, United Way of Pelham, NY, the Pelham Art Center, the Federal Bar Council, the Village Board of Pelham Manor, the International Senior Lawyers Project and the Point O’Woods, NY Association. He served for 15 years as a volunteer coordinator for the AmeriCares/HomeFront home rehabilitation project, and for 28 seasons as a travel soccer coach.

Mr. Kiernan is a past President of the New York City Bar Association, where he previously served on the Executive Committee and chaired several other committees. He has also chaired numerous committees of the Litigation Section of the American Bar Association and the Federal Bar Council. He is the Chair of the New York Chief Judge’s Advisory Committee on Alternative Methods of Dispute Resolution and a member of the New York State Permanent Commission on Access to Justice and the Legal Services Corporation’s Disaster Task Force, and he has previously served as Co-Chair of the New York Chief Judge’s Task Force on Hurricane Disaster Relief and as a member of Chief Judge’s Committee on Non-Lawyers and the Justice Gap. He was a speaker at annual ALI-ABA Seminars on Accountants’ Liability for 12 years and has spoken at numerous other seminars and written numerous articles on substantive issues in litigation, approaches to resolving complex disputes and discovery. He is the co-editor of The Litigation Manual (ABA, 3rd ed., 1999) and a contributing author of New York Business Litigation (ALM 2014) and Commercial Litigation in New York State Courts (Thomson Reuters, 4th ed., 2015) and has been an Adjunct Professor at New York University Law School.

Mr. Kiernan joined Debevoise in 1981 and became a partner in 1988. He received his B.A. in 1976 magna cum laude from Harvard and his J.D. in 1980 magna cum laude from Harvard Law School, where he was an editor of the Harvard Law Review. From 1980–1981, Mr. Kiernan served as a law clerk to the Hon. Walter R. Mansfield, U.S. Court of Appeals for the Second Circuit.


  • Harvard Law School, 1980, J.D.
  • Harvard University, 1976, B.A.

Bar Admissions

  • New York