Thomson was called to active duty on March 24, 1941 and commanded the Second Battalion, Three Hundred and Sixty-second Infantry Regiment, Ninety-first Division. He was released from active duty as a lieutenant colonel on January 24, 1946. He had been admitted to the bar in 1941 and commenced the practice of law in Cheyenne in February 1946; he was a delegate to the 1952 Republican National Convention in Chicago and was a member of the Wyoming House of Representatives from 1952 to 1954.