In 1928, Galvin enlisted as a private in the 9th Infantry Division. He went on active duty in 1940. General George Patton placed Galvin in the lead tank of his Flying columns. Galvin spent 280 days as a tank officer on the front line. He fought in Normandy on D-Day and at the Siege of Bastogne and was awarded the Silver Star, two Bronze Stars, the Legion of Merit, and the Croix de Guerre. He was wounded in action in 1944. From 1944 to 1945, Galvin was the operations chief of the 6th Armored Division. He was responsible for training the division and drawing up battle plans. Galvin later spent fifteen months as the 6th's intelligence chief. General George Marshall described Galvin as "the outstanding reservist in the war" before the United States Congress. He finished the war with the rank of colonel.