Leathley was commissioned as a second lieutenant in the 5th Battalion, Prince of Wales's Leinster Regiment in early 1914, being confirmed in his rank on 24 July, and promoted to lieutenant on 24 October. Leathley resigned his Special Reserve commission on 17 February 1915, entering the Royal Military College as a "Gentleman Cadet", and after passing out was commissioned as a second lieutenant in the Royal Inniskilling Fusiliers on 16 June 1915. He was promoted to lieutenant on 11 April 1916. Leathley was seconded to the Royal Flying Corps on 6 March 1917, and posted to No. 57 Squadron RFC, where he gained his first aerial victory, flying with Lieutenant C. S. Morice in a F.E.2d, by driving down out of control an Albatros D.III fighter at Buissy on 30 April, before he was officially appointed a flying officer on 8 June. All his subsequent victories were over Albatros D.V fighters and were made with Major Ernest Joy as his pilot flying an Airco DH.4. Joy and Leathley accounted for two enemy aircraft on 28 July, and then five in August; one each on the 16th and 20th, and three on the 17th. On 26 September 1917 Leathley was awarded the Military Cross. His citation, gazetted on 8 January 1918, read: By that time Leathley was training to become a pilot himself, having been granted Royal Aero Club Aviator's Certificate No. 5455 after soloing a Maurice Farman biplane at the Military School, Ruislip, on 26 November 1917, and was appointed a flying officer on 11 May 1918.
List of aerial victories
Inter-war career
Leathley remained in the RAF post-war, being granted a permanent commission with the rank of lieutenant on 1 August 1919. He served as an instructor at No. 4 Flying Training School at RAF Abu Sueir, Egypt, from 20 May 1921, receiving promotion to flight lieutenant on 30 June 1922, until posted to the Aircraft Depot on 21 June 1924. On 4 November 1924 Leathley was placed on half-pay, and went onto the retired list on 27 January 1925.
World War II
On 5 October 1940 Leathley received an emergency commission as a lieutenant in the King's Royal Rifle Corps. He was promoted to the war substantive rank of captain, and granted the temporary rank of major on 5 November 1941 while being "specially employed".
Personal life
In January 1920 Leathley became engaged to Ida Foster from London, and they were married at St George's, Hanover Square, on 27 April.
