Second Lieutenant Cosnahan was a member of the 21st Infantry Regiment, 24th Infantry Division. He was Killed in Action while fighting the enemy near Kunu-ri, South Korea, on 12 July 1950.

Cosnahan was awarded 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.