After the war, Priestley was promoted to acting major on 24 January 1919, and was seconded to the War Office that year to write the history of the signal service. He also wrote "Breaking the Hindenburg Line", an account of 46 (North Midland) Division's spectacular attack during the Battle of St Quentin Canal. During April–May 1919, he was a staff officer to the Signal Officer-in-Chief, with the temporary rank of major. He relinquished his temporary commission on 17 November 1920, reverting to the permanent rank of lieutenant in the Territorial Force. From 19 February 1921, he again held the temporary rank of major in the reserves, in the 3rd London, Royal Corps of Signals. On 6 July 1921, he was commissioned a lieutenant in the Cambridge University Contingent (Senior Division), Officers Training Corps. He was promoted to captain on 21 June 1922 and resigned his commission on 30 June 1926, retaining the rank of major.