Fagan was commissioned a second lieutenant in the Marines in January 1942. Fagan served as commanding officer of Company G, Second Battalion, 9th Marine Regiment, 3rd Marine Division. He fought in the Bougainville, Guadalcanal, New Zealand, Kwajalein, Eniwetok, and Guam campaigns. He was severely wounded in hand-to-hand combat on Iwo Jima on February 26, 1945; he was evacuated and died the following day. Fagan was twice awarded the Navy Cross: once for his actions at the Battle of Guam, the other for his actions at the Battle of Iwo Jima. He received the second award posthumously.