Abandoning his university studies, Stephens enlisted in the Australian Army in July 1915 at Perth. He embarked from Alexandria for Marseilles in June 1916; entrained to the Northern Front, Ypres area, for some weeks front line service; then by road to R. Somme Offensive area for front line service with the 4th Machine Gun Company of the 12th Brigade; became a 2nd Lieutenant; in September he returned to the Northern Front. There he answered a call for volunteers to join the Royal Flying Corps.
Stephens served as an instructor before being posted to 41 Squadron on 16 March 1918. Flying the S.E.5a, he shared his first victory with Fred McCall on 28 June 1918. By the end of the war, he scored twelve more victories and became a flight commander. In the 1930's, Stephens was a commercial pilot for Qantas and was also a pilot for the Kingsford Smith, Charles Ulm company (the original) Australian National Airways until it closed down.