Semple attended the Brooks School in North Andover, Massachusetts, graduating in 1940. He then attended Yale University, but left in 1941 to join the American Field Service in North Africa, where his boyish beard earned him the nickname "the goat". Aged 19, he was awarded the Médaille militaire and Croix de Guerre for his service as a volunteer ambulance driver with the Free French forces in Libya. Wounded in action at Bir Hakeim, he returned to the United States where he was drafted into the army, serving as an intelligence officer in Europe.