Staff Sergeant Mast was a crew member on a RB-29A reconnaissance Superfortress with the 91st Strategic Reconnaissance Squadron, U.S. Air Force. On 4 July 1952, while on a photo reconnaissance mission over Sinanju, North Korea possibly China, his aircraft was shot down by a MiG. He was listed as Missing in Action and was presumed dead on 28 February 1954. His remains were not recovered. His name is inscribed on the Courts of the Missing at the Honolulu Memorial.

Mast was awarded the Air Medal, 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.