Early in 1938,Sandy Johnstone was a civilian navigation instructor at Scottish Aviation. In August 1939, he was called to full-time service with No. 602 Squadron. After participating in engagements with lone German raiders off the Scottish coast in late 1939-early 1940, he was given command of 602 Squadron in July 1940 at the age of 24. By this time Johnstone had already claimed a Heinkel He 111, a Junkers Ju 88 and a Dornier Do 17 damaged. The squadron was transferred south to RAF Westhampnett in West Sussex, where it was stationed throughout the Battle of Britain. His tally after the battle (and his final wartime score) was 7 (and 2 shared) aircraft destroyed, 1 probable, and 6 (and 1 shared) damaged. He was awarded the Distinguished Flying Cross for his actions.