William Tiernan joined the Platoon Leaders Class in 1953 while an undergraduate student at Indiana University, and was commissioned upon his graduation in 1954. During the Cuban Missile Crisis in 1962, he was Commanding Officer of Company F, 2d Battalion, 1st Marines, and participated in the deployment of Battalion Landing Team 2/1 to Guantanamo Bay, Cuba. He retired as a United States Marine Corps Brigadier General.