Kerby joined the Royal Naval Air Service in February 1915 as a probationary flight sub-lieutenant, being confirmed in his rank on 21 March. He was granted Royal Aero Club Aviators Certificate No. 1214 after soloing a Grahame-White biplane at the Grahame-White School at Hendon on 5 May. On 12 June Kerby was posted to No. 3 Squadron, under Commander Charles Samson, to serve in the Gallipoli Campaign. He was wounded on 26 November, and after a year recuperating, was reinstated on 27 November 1916. He was then posted to No. 9 Squadron in France, where on 24 March 1917 he gained his first aerial victory, flying a Sopwith Pup, sharing in the destruction of an enemy seaplane at Wenduine. Promoted to flight lieutenant on 1 April, he then returned to No. 3 Squadron, now also based in France, where he gained six more aerial victories between 22 April and 27 May. In August, while serving in the Walmer Defence Flight, he destroyed two Gotha G.IV heavy bombers. He was subsequently awarded the Distinguished Service Cross, which was gazetted on 2 November 1917. His citation read: On 31 December he was promoted to flight commander. On 1 April 1918, the Royal Naval Air Service was merged with the Army's Royal Flying Corps to form the Royal Air Force, and Kerby joined the RAF with the rank of lieutenant. On 18 May he was appointed a temporary major, to serve as Officer Commanding and Instructor at No. 4 Fighter School, RAF Freiston.
List of aerial victories
Inter-war career
Kerby was awarded the Air Force Cross in the New Years Honours of 1 January 1919. He was again appointed an acting major on 1 May, and on 1 August was granted a permanent commission in the RAF with the rank of captain. He served on the staff of the No. 1 School of Technical Training at RAF Halton from December 1919, until sent to the RAF Staff College at Andover on 3 April 1922. He was one of the first students at the Staff College, which had only just opened under the command of Air Commodore Robert Brooke-Popham. The Directing Staff included Air Commodore Robert Clark-Hall, Group Captain Philip Joubert de la Ferté, Wing Commanders Wilfrid Freeman and Charles Edmonds, and Squadron Leader Bertine Sutton, while Kerby's fellow students included Wing Commanders Jack Baldwin and Richard Peirse, Squadron Leaders Conway Pulford, Keith Park, Wilfred McClaughry and Charles Portal, and Flight LieutenantsEric Betts and John Cole-Hamilton. On completion of the Staff College course he was posted to the Air Staff of No. 3 Group on 4 April 1923, then to the staff of No. 3 Wing, India, on 14 September. Kerby was promoted to squadron leader on 10 October 1928, before finally returning to England to be posted to the RAF Depot at Uxbridge on 6 April 1929. On 1 November he was posted to serve on the staff of the Directorate of Operations and Intelligence in the Air Ministry, before finally returning to flying on 4 April 1934 as Officer Commanding No. 504 Squadron, a Special Reserve Squadron of the Auxiliary Air Force operating Westland Wallace bombers from RAF Hucknall. He was promoted to wing commander on 1 July 1935, leaving No. 504 Squadron on 5 August. From 5 October he was a supernumerary officer on the staff of RAF Far East, and from 16 November 1936 served as Air Attaché at the British Embassy in Beijing and Shanghai. On 1 January 1939 Kerby was promoted to group captain, returning to the No. 1 RAF Depot on the 26th.
Kerby retired from the RAF on 28 June 1946, retaining the rank of air vice marshal. On 15 October 1946 he was granted permission to wear the insignia of an Officer of the Legion of Merit conferred by the United States, followed by Commander of the Legion of Merit on 29 July 1947. He died on 8 June 1963.