William Dewitt Mitchell was born in Winona, Minnesota, on September 9, 1874. He attended the University of Minnesota, earning his A.B. in 1895, and his L.L.B in 1896.

Mitchell began a private law practice in St. Paul but soon saw action as an infantry officer during the Spanish-American War; he would also fight in World War I. On June 4, 1925, President Calvin Coolidge appointed Mitchell solicitor general. Mitchell remained at that post until President Herbert Hoover appointed him attorney general, a position Mitchell held from 1929 to 1933.

Following his time in the cabinet, Mitchell served as chairman on the Committee of Federal Rules of Civil Procedure, and as chief counsel to the joint committee investigating the Pearl Harbor attack.

William Mitchell died in Syosset, New York, on August 24, 1955.