John K. Zwerling is a prominent criminal defense attorney based in the Washington, D.C. metropolitan area, known for his work on high-profile federal cases involving espionage, corruption, and political scandals. As the founding partner of the law firm Zwerling & Kemler, P.C., in Alexandria, Virginia, he has represented a diverse array of clients, including government officials, lobbyists, and business executives. Zwerling's legal career spans over five decades, during which he has earned a reputation for aggressive defense strategies in complex federal prosecutions, often challenging government prosecutions on constitutional grounds. Earlier in his career, he served as an Assistant United States Attorney in the Eastern District of Virginia, gaining invaluable experience in federal litigation before transitioning to private practice.

Zwerling rose to national prominence through his representation of Keith Weissman, a senior lobbyist for the American Israel Public Affairs Committee (AIPAC), in the 2005 AIPAC espionage case. The case accused Weissman and colleague Steven J. Rosen of conspiring to transmit classified national defense information to unauthorized persons, stemming from interactions with Pentagon analyst Larry Franklin. Zwerling's defense efforts contributed to the eventual dismissal of all charges against his client in 2009, following a protracted legal battle that highlighted issues of free speech and overreach in espionage prosecutions.

Beyond the AIPAC case, Zwerling has handled numerous notable defenses, including representing clients in investigations related to public corruption and white-collar crime, as well as matters related to the Iran-Contra affair, espionage trials, and various corruption investigations tied to political figures. His practice emphasizes federal criminal defense, and he has been involved in cases before the U.S. Supreme Court and various appellate courts. Currently, Zwerling is a partner at Zwerling, Leibig & Moseley, PC, where he continues to handle significant white-collar crime and national security cases. His expertise in federal criminal law has made him a go-to attorney for individuals facing scrutiny from agencies like the FBI and DOJ.

Zwerling graduated from the University of Virginia with a B.A. in History in 1969 and earned his J.D. from the University of Virginia School of Law in 1972. He is admitted to practice in Virginia and the District of Columbia, and his firm is recognized for its focus on appellate work and trial advocacy. Throughout his career, he has contributed to legal scholarship, commentary on criminal justice issues, and legal education through lectures and writings on criminal procedure, emphasizing the importance of constitutional rights in high-stakes litigation.