Sergeant Monaghan was a member of Company C, 1st Battalion, 32nd Infantry Regiment, 7th Infantry Division. He was Killed in Action while fighting the enemy in North Korea on 25 March 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, Sergeant Monaghan was awarded the Bronze Star Medal, the Purple Heart, the Combat Infantryman's Badge, 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.