In 1940, after Stompanato's freshman year at Woodstock High School, his father sent him to Kemper Military School for boys in Boonville, Missouri. In 1942, he graduated at the age of 17. In 1943, Stompanato joined the U.S. Marines serving with the 1st Service Battalion, 1st Marine Division. He served in the South Pacific theater, in Peleliu and Okinawa, and then served in China. Stompanato left the Corps in March 1946, being discharged in China.