After completing his education, he worked as an accounts administrator until applying to join the Royal Canadian Air Force (RCAF) in October 1940.
[1] Once Mitchner's flying training was completed, in July 1941 he was sent to the United Kingdom to serve with the Royal Air Force as a non-commissioned officer.
At the time Mitchner joined the squadron, it was regularly flying on offensive operations to Europe but over the winter months, it was engaged in convoy patrols.
Mitchner was commissioned as a pilot officer on 12 January 1943 and five days later achieved his first aerial victory, a Focke-Wulf Fw 190 fighter that was damaged near Cabourg.
In March the squadron moved north to Digby for a period of less intensive duties and training, but returned to offensive operations a few months later.
[1][5] On 24 September 1943, Mitchner, now promoted to flight lieutenant, shared in the destruction of two Fw 190s, one confirmed and the other deemed a probable, both over the Beauvais region.
The published citation read: This officer has completed a large number of sorties, including several very successful attacks on shipping.
In combat, Flight Lieutenant Mitchner has destroyed 3 enemy aircraft.Mitchner achieved no further aerial victories during the remainder of his service with the squadron, which spent the winter of 1943–1944 at Digby.
Returning to the United Kingdom, after a period of duty at the RAF station at Tealing, he resumed operational flying with a posting as a flight commander at No.
[1] This was another RCAF fighter squadron which, since shortly after the invasion of Normandy, operated Spitfires from airfields in France as part of the Second Tactical Air Force.
416 Squadron, which by this time was engaged in armed reconnaissance sorties as the Allied ground forces advanced through Belgium and into Germany.
The published citation read: This officer has completed many sorties against the enemy since being awarded the Distinguished Flying Cross.
One day in April, 1945, Squadron Leader Mitchner led two armed reconnaissances, resulting in the destruction of 45 motor transport, whilst more than another 100 were damaged.
[1][8] In recognition of valuable services rendered 'in connection with the war', the Queen of the Netherlands awarded Mitchner the Vliegerkruis (Flying Cross), the announcement being made in The London Gazette on 1 January 1946.