Assistant U.S. Attorney General in charge of the Criminal Division, U.S. Department of Justice, 1935-1939.
Sponsored the Atomic Energy Act of 1946 (McMahon Act), establishing the civilian United States Atomic Energy Commission to supervise U.S. nuclear technology.
United States Senator from Connecticut, 1945-1952.
Co-chair, Joint Senate House Committee on Atomic Energy, 1949-1952.