Corporal Smithson was a member of Company F, 2nd Battalion, 1st Marines, 1st Marine Division. He was listed as Missing in Action while fighting the enemy in Korea on 27 December 1952. He was presumed dead on 29 December 1953. His remains were not recovered. His name is inscribed on the Courts of the Missing at the Honolulu Memorial.

Smithson was awarded the Purple Heart, the Combat Action Ribbon, 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.