Carnall joined the RAF as an apprentice in January 1929 at sixteen years old. He qualified as a fitter in December 1931, and was later offered the opportunity to volunteer for pilot training, which he completed in early 1937. He was posted to 111 Squadron at Northolt, northwest London, one of the first squadrons to have Hurricanes. He was flying a Hurricane as war broke out, taking part in defensive patrols and reconnaissance sorties.

In August 1940, Carnall was shot down during a fierce air battle over Kent. He crashed near Paddock Wood, surviving the impact, but the flames that took over the cockpit left him with severe burns. He was treated at hospital in Tunbridge Wells, before being transferred to QVH, where he remained for more than a year.

In 1942, Carnall was commissioned as a Pilot Officer, and became a flight instructor. In the spring of 1944, he was posted to 684 Squadron, a Mosquito photo-reconnaissance unit based in Calutta (now Kolkata), where he remained for the rest of the war.

He left the RAF In 1963.