After graduating from the University of Virginia Law School in 1940, where he was editor-in-chief of the Law Review, Mr. Caplin served as a law clerk to U.S. Circuit Judge Armistead M. Dobie. He then practiced law in New York City from 1941 to 1950, with time out for military service in the U.S. Navy. During the Normandy invasion, he served as U.S. Navy beachmaster and was cited as a member of the initial landing force on Omaha Beach. In 1950, Mr. Caplin returned to the University of Virginia as a professor of law, specializing in tax and corporate law and publishing extensively in both of these fields. Following President John F. Kennedy's election, Mr. Caplin served on the President's Task Force on Taxation and, in January 1961, was appointed U.S. Commissioner of Internal Revenue. Robert F. Kennedy (1951) and Edward M. Kennedy (1959) had been among his law students. He remained in that post until July 1964, when he resigned to form the law firm of Caplin & Drysdale based in Washington, D.C. On leaving the U. S. government, he received the Alexander Hamilton Award, the highest award conferred by the Secretary of the Treasury, for his "distinguished leadership."

Mr. Caplin has deep commitments to public service, teaching, and the educational process itself. He was a visiting professor in tax law at UVA for 22 years, retiring as a professor emeritus in 1987. Mr. Caplin has served as Trustee of many educational and charitable organizations, including the Board of Visitors, University of Virginia (1992-1997); UVA Law School Foundation; and Miller Center of Public Affairs. Caplin has been a generous benefactor to UVA, especially to the Law School. His visionary gifts have funded the Law School's Caplin Auditorium, the Daniel Caplin Professorship, the Mortimer Caplin Public Service Scholarship, the Mortimer Caplin Public Service Award and the Mortimer Caplin Public Service Center as well as several major improvements to the school's facilities. Most recently, he was presented with the Thomas Jefferson Medal in Law, the University of Virginia’s highest honor. He is Professor Emeritus at UVA Law School. As former Law School Dean Robert Scott once noted, "Mortimer Caplin has done it all."