Born in Dallas, Texas, Clark served in the United States Marine Corps in 1945 and 1946, then earned a B.A degree from the University of Texas in 1949, an M.A and a J.D from the University of Chicago in 1950. He was admitted to the Texas bar in 1951, and to practice before the Supreme Court of the United States in 1956. From 1951 to 1961 Clark was an associate and partner in the law firm of Clark, Reed and Clark. He served in the United States Department of Justice as Assistant Attorney General of the Lands Division from 1961 to 1965, and as Deputy Attorney General from 1965 to 1967. Clark was director of the American Judicature Society in 1963. From 1964 to 1965 he was national president of the Federal Bar Association. On March 2, 1967, President Johnson appointed him Attorney General of the United States. He served in that capacity until January 20, 1969.
Clark is well known for representing Nazi emigres to the United States, including those believed to be war criminals.
Clark represented Liberian political figure Charles Taylor during his 1985 fight against extradition from the United States to Liberia.
Clark is affiliated with VoteToImpeach, an organization advocating the impeachment of President George W. Bush. He has been an opponent of both Gulf Wars and, briefly, a supporter of Lyndon LaRouche. He is the founder of the International Action Center (an offshoot of the Socialist Workers Party).