Thomas Clark was born on September 23, 1899, in Dallas, Texas. He received a bachelor's degree from the University of Texas in 1921 and a law degree from the same institution the following year. Clark practiced law and was active in local government until 1937, when he was named to a position in the Department of Justice, working mostly in the Antitrust Division.

In 1945, President Harry S. Truman appointed Clark attorney general, a post Clark would hold until 1949. Clark became one of Truman's key domestic advisers and a staunch supporter of Truman's anticommunist loyalty programs. The President nominated Clark as a Supreme Court justice in 1949, and the appointment was secured by an overwhelming Senate majority.

Clark remained an associate justice until 1967 and ruled on many important issues during the Truman, Eisenhower, Kennedy, and Johnson administrations. For the most part, he took a conservative line on cases dealing with loyalty to the government or national security, but he was a moderate on those involving criminal rights. Thomas Clark died on June 13, 1977.