The son of Martin Seth-Smith, Cadet Kenneth Arthur Seth-Smith, of the 3rd Battalion, Northumberland Fusiliers, was promoted to 2nd Lieutenant (on probation) on 24 March 1915 and confirmed in rank on 2 November 1915. He was seconded to the Royal Flying Corps as a Flying Officer (Observer) on 12 May 1917. Posted to 70 Squadron, he scored his first victory from the backseat of a Sopwith 1½ Strutter in May 1917. By December, he was a pilot and scored six more victories flying the Sopwith Camel. He was downed by Max von Müller on 16 December 1917 and wounded in action on 23 March 1918. On 19 April 1918, he was promoted to Lieutenant.
Supplement to the London Gazette, 19 February 1917 (29952/1740)
North'd Fus.—Lt. Kenneth Arthur Seth
Smith, from a Spec. Res. Bn., to be 2nd
Lt. 20th Feb. 1917, with seniority as from
19th Oct. 1916, and precedence next above
H. F. Attwater, and to retain his higher
rank and seniority in his present unit until
ordered to join a Regular Unit."