Leathley was commissioned as a second lieutenant (on probation) in the 5th Battalion,
Prince of Wales's Leinster Regiment (Royal Canadians) (part of the
Special Reserve) 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 (observer) on 8 June. His citation,
gazetted on 8 January 1918, read: :Lieutenant Forde Leathley, Royal Inniskilling Fusiliers and Royal Flying Corps. ::"For conspicuous gallantry and devotion to duty in making photographic reconnaissances and in fighting enemy aircraft. Since April he has taken part in numerous combats, during which seven hostile machines have been driven down and destroyed either by him or his Pilot, and although attacked by superior numbers of the enemy, his skill and offensive spirit have enabled him to carry out photographic reconnaissances." 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,
List of aerial victories ==Inter-war career==