John Thornell was born on April 19, 1921, in Stoughton, Massachusetts. He enlisted in the U.S. Army Reserve on June 11, 1940, and then served on active duty with the 1st Infantry Division at Fort Ethan Allen, Vermont, from July 2, 1940, to January 1942, when he entered the Aviation Cadet Program of the U.S. Army Air Forces. Thornell was commissioned a 2d Lt and awarded his pilot wings at Craig Field, Alabama, on February 16, 1943, and after P-47 Thunderbolt training he was assigned to the 328th Fighter Squadron of the 352nd Fighter Group in England in July 1943. Lt Thornell was credited with the destruction of 17.25 enemy aircraft in aerial combat plus 2 damaged, and another 3 on the ground while strafing enemy airfields, between January and June 1944. He returned to the U.S. in July 1944 and after the war he served with the 1st Tactical Reconnaissance Squadron in Furth, Germany, from 1946 to February 1947, followed by service in the Air Force Reserve from February 21, 1947 to March 22, 1948. After returning to active duty, he served with the 4th Tactical Fighter Wing at Andrews AFB, Maryland, and then was an adviser to the Turkish Air Force from 1949 to 1951. Thornell then served as Operations Officer for the 89th Fighter Group at Bedford, Massachusetts, until 1955, followed by service in the Atomic Plans Office with Headquarters 3rd Air Force in England from August 1956 to August 1960. His next assignment was as Flying Safety Officer with the 4530th Combat Crew Training Wing and then with the 3525th Pilot Training Wing at Williams AFB, Arizona, from August 1960 to August 1962, and then as commander of the 3526th Pilot Training Squadron, also at Williams, until July 1964. Col Thornell was Chief of the Flight Training Division and then Chief of the Flight Safety Division with Headquarters Air Training Command at Randolph AFB, Texas, from July 1964 to July 1967. His final assignment was as Chief of the Fighter Branch with the Inspector General Group at Norton AFB, California, from July 1967 until his retirement from the Air Force on August 1, 1971. John Thornell died on September 3, 1998, and was buried at Riverside National Cemetery in Riverside, California.

His Distinguished Service Cross Citation reads:

For extraordinary heroism in action against the enemy, 8 May 1944. On this date, Lieutenant Thornell, with complete disregard for the odds against him, led a flight against a vastly superior force of enemy fighters in the vicinity of Nienburg, Germany, and dispersed the enemy attempting to intercept a friendly bomber formation. Determined to destroy the enemy, accompanied by two other friendly fighters he attacked three enemy fighters and by courageous flying and skillful gunnery destroyed two of them. Later, Lieutenant Thornell was attacked by a lone enemy fighter whom he outmaneuvered and destroyed, bringing his total for the day to three enemy airplanes destroyed. The outstanding courage, coolness, and skill displayed by Lieutenant Thornell upon this occasion reflect highest credit upon himself and the Armed Forces of the United States.