Sergeant First Class Hynes was a member of Battery C, 38th Field Artillery Battalion, 2nd Infantry Division. He was Killed in Action while fighting the enemy near Kunu-ri, North Korea, on 30 November 1950. His remains were not recovered. His name is inscribed on the Courts of the Missing at the Honolulu Memorial.

Hynes 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.