First Lieutenant McMullan was a member of Battery C, 15th Field Artillery Battalion, 2nd Infantry Division. He was taken Prisoner of War while fighting the enemy in Korea on 13 February 1951 and was killed by an air raid on Suan Beam Camp on 22 April 1951. His remains were not recovered. His name is inscribed on the Courts of the Missing at the Honolulu Memorial.

McMullan was awarded the Purple Heart, the Prisoner of War Medal, 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.