Stevens was commissioned a second lieutenant in the infantry on 1914-09-22. He served briefly in the 9th Service Btn., before being seconded for service with the Motor Machine Gun Service on 1914-11-21. He was promoted to lieutenant on 1915-03-01. Stevens joined the Royal Flying Corps as a gunner on 1916-06-19, but soon trained as a pilot, and was posted to No. 20 Squadron RFC, to fly the F.E.2d two seater fighter, where he gained his first victory. On 1917-06-09 he was appointed a flight commander and Captain, and gained his remaining four victories, between 17 May and 16 August 1917. His gunners included fellow ace Lieutenant William Cambray. On 1919-03-21, Stevens was transferred to the RAF unemployed list, and on June 3, in the King's Birthday Honours, he was made an Officer of the Order of the British Empire "in recognition of distinguished services rendered during the War".