He left the private sector to seek public office and was elected as Arkansas Attorney General in 1970. He was elected two years later to Congress. He defeated fellow Democrats Richard S. Arnold of Texarkana and Richard Mays, El Dorado in the primary, with no Republican in the race.
Thornton went on to serve three terms in the House. He distinguished himself as a member of the Judiciary Committee, which considered articles of impeachment against U.S. President Richard Nixon. He was among three southern Democrats and four moderate Republicans who drafted the articles adopted by the committee.