In 1942, Thayer left college at the end of his sophomore year and joined the Army. He entered service at the Presidio of Monterey as an infantry private, but was selected for Officer Candidate School shortly after induction. He was trained as a mine platoon leader at Fort Benning, Georgia. He was commissioned a second lieutenant on October 3, 1944, and assigned to a front line anti-tank company in the 71st Infantry Division, which was operating in Normandy at the time.
