Clark served as Texas National Guard infantryman in 1918; afterward he studied law, receiving his law degree from the University of Texas at Austin in 1922 and setting up practice in his home town of Dallas from 1922-1937. He resigned from private practice for a period to serve as civil district attorney for the city from 1927-32.
Clark, a Democrat, joined the Justice Department in 1937 and served as civilian coordinator for the forced relocation of Japanese-Americans in California and elsewhere during the opening months of World War II (see Japanese internment). Later, he headed the antitrust and criminal divisions at Justice.
Appointed Attorney General by Truman in 1945, Clark was appointed to the court in August 1949, filling the vacancy left by the death of Frank Murphy.
While on the court, Clark often took a decidedly anti-Communist stance during the "Red Scare." In other areas, he is noted for writing the majority opinion in such cases as Mapp v. Ohio and Abington School District v. Schempp, both considered cases affirming basic constitutional rights. Clark supported the end of racial segregation, siding with the majority in Brown v. Board of Education and Sweatt v. Painter.
Clark retired from the court on June 12, 1967, to avoid conflict of interest when his son, Ramsey Clark, was appointed Attorney General. He was succeeded in his post by Thurgood Marshall. After his retirement he served as a visiting justice in the US Courts of Appeals, as director of the Federal Judical Center, and as Chair of the Board of Directors for the American Judicature Society.