He enlisted in the Aviation Cadet Program of the U.S. Army Air Corps on February 9, 1940, and was commissioned a 2nd Lieutenant and awarded his pilot wings at Kelly Field, Texas, on October 5, 1940.

He began flying frequent escort missions and destroyed a Fw 190 and Bf 109 on November 11, for which he received the Distinguished Service Cross.

During a bomber escort near Ans, Belgium on 20 January 1944, his flight encountered four twin-engined Messerschmitt Bf 110s.

During World War II, Duncan was credited with destroying 19.5 enemy aircraft in the air, 1 probable kill, and 7 damaged.

He then served as an instructor with the Air National Guard until June 1949, when he became a White House Liaison Officer.

Glenn Duncan's ribbons, including retroactive awards:-[4] Command pilot badge Citation: The President of the United States of America, authorized by Act of Congress, July 9, 1918, takes pleasure in presenting the Distinguished Service Cross to Lieutenant Colonel (Air Corps), [then Major] Glenn Emile Duncan, United States Army Air Forces, for extraordinary heroism in connection with military operations against an armed enemy while serving as Pilot of a P-47 Fighter Airplane in the 353d Fighter Group, EIGHTH Air Force, in aerial combat against enemy forces over enemy-occupied Europe while leading a group of fighter aircraft on 11 November 1943.

Although his gas supply was dangerously low, he engaged the enemy and dispersed them, thereby saving the Fortress and its crew.

The action of Lieutenant Colonel Duncan reflect the highest credit upon himself and the armed forces of the United States.