Lock joined the RAFVR in 1939, aged 19, to begin a part-time training course. He qualified as a pilot in May 1939 and joined 41 Sqn in June 1940.
In September, the squadron moved south, to Hornchurch and began operational flying in the Battle of Britain, and in the first week Lock shot down nine aircraft. He became an official Ace on his second encounter with the enemy. It rose to 15 by October. On the day his total rose to 22, 17 November, his plane was hit by cannon shells from a Bf 109. They severely injured his right arm and both legs, knocked the throttle fully open and severed the throttle control lever. He was forced to cut the engine before crash landing in a field near Alderton, Suffolk. He was trapped in the cockpit for two hours before he was found and rescued by two soldiers.
Over the next three months, he had 15 operations. He was discharged from QVH in April 1941 and posted to 611 Sqn. He shot down three more aircraft before being lost over Calais on 3 August.

Lock was the youngest pilot to receive the DSO at the time, and the first person to receive three decorations at the same time from King George VI. He is also credited with being the highest-scoring Ace of the Battle of Britain.