Corporal MacMillin was a member of Company F, 2nd Battalion, 5th Marines, 1st Marine Division. On 22 August 1952, he was on a patrol forward of the "Jamestown" front line in an area near the "Hook." His patrol was ambushed and he was killed by small arms fire.

MacMillin 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.