First Lieutenant Tiernan was the pilot of a F-84E Thunderjet fighter with Headquarters Squadron, 49th Fighter-Bomber Group. On 17 March 1952, while on a combat mission, his aircraft received a direct hit by anti-aircraft fire, crashed and burst into flames. He was listed as Missing in Action and was presumed dead on 31 December 1953. His remains were not recovered. His name is inscribed on the Courts of the Missing at the Honolulu Memorial.

For his leadership and valor, First Lieutenant Tiernan was awarded the Distinguished Flying Cross, the Air Medal with Oak Leaf Cluster, the Purple Heart, the Korean Service Medal, the United Nations Service Medal, the National Defense Service Medal, the Korean Presidential Unit Citation and the Republic of Korea War Service Medal.