Corporal Coates was a member of Battery B, 503rd Field Artillery Battalion, 2nd Infantry Division. He was taken Prisoner of War by the enemy near Kunu-ri, North Korea on 30 November 1950 and died as a prisoner on 30 January 1951. His remains were not recovered. His name is inscribed on the Courts of the Missing at the Honolulu Memorial.

Coates was awarded 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.