Leonard joined the RAF in 1940, flying Whitley bombers with No. 102 Sqn. He soon demonstrated that he was a calm and efficient captain, a quiet but determined leader.

On the night of 12 November 1940 his aircraft was hit by flak (anti-aircraft fire) near Cologne and a reconnaissance flare exploded in the fuselage. Leonard successfully brought the damaged aircraft back to base, for which he was awarded the first of his three Distinguished Service Orders.