Leask was transferred to the RAF's Technical Branch on 24 April 1940, and was appointed an acting air commodore to serve as Air Officer Commanding (AOC) No. 43 (Maintenance) Group (part of RAF Maintenance Command) from 20 November. On 1 January 1941 he received a mention in despatches from his Air Officer Commanding-in-Chief. On 1 June 1941 he was promoted to the temporary rank of air commodore, and on 21 August 1942 he was appointed an acting air vice marshal. A year later, on 21 August 1943, his temporary appointment as an air commodore was made war substantive. He became AOC, No 24 (Technical Training) Group (part of RAF Technical Training Command) on 10 January 1944.
