Early in 1939 John Edmondson enlisted as a reserve soldier in the 4th Battalion (The Australian Rifles) Citizen Military Forces.
On the day after the war was declared in September 1939, he volunteered for fulltime duty. Upon transfer to the A.I.F. early in 1940, he trained at the newly-established Ingleburn Military Camp.
John Edmondson sailed for the Middle East in 1940. His unit was engaged in the Battle of Tobruk, an action in which he lost his life but gained the Victoria Cross (V.C.). The V.C. Honour Roll cites his bravery in the following terms: "On the night of April 13-14, 1941, a party of [enemy] infantry broke through the wire defenses at Tobruk and established themselves with at least six machine guns, mortars, and two small field pieces. It was decided to attack them with bayonets, and a party consisting of one officer, Corporal Edmondson, and five privates, took part in the charge. During the counter-attack Corporal Edmondson was wounded in the neck and stomach but continued to advance under heavy fire, and killed one of the enemy with his bayonet, Later his officer had his bayonet in one of the enemy and was grasped about the legs by him, when another attacked him from behind. He called for help, and Corporal Edmondson, who was some metres away, immediately came to his assistance and in spite of his wounds killed both of the enemy.
This action undoubtedly saved his officer’s life. Shortly after returning from this successful counter-attack, Corporal Edmondson died of wounds. His actions throughout the operations were outstanding for resolution, leadership and conspicuous bravery.
The posthumous award was approved by his "Majesty King George VI in July 1941. It was presented to John Edmondson’s parents at Admiralty House, Sydney on 27 September 1941.