Harrison was commissioned as second lieutenant in the Corps of Royal Engineers on 19 November 1915. He served in France and German West Africa, and was promoted to lieutenant on 4 October 1916.
He was seconded to the Royal Flying Corps, and on 17 May 1918 was appointed an observer officer with the rank of second lieutenant, though with the honorary rank of lieutenant. He was posted to No. 98 Squadron, flying in the DH.9. Harrison gained his first victory on 21 April 1918 with Lieutenant A. M. Phillips as pilot, by driving down 'out of control' a Fokker Dr.I over Bailleul. His second came on 8 May, with Lieutenant N. C. MacDonald, when he set on fire an Albatros D.V over Menen–Wervicq. Flying with Lieutenant Frederick Wilton, he accounted for two more Fokker Dr.I's on 16 July, and a Pfalz D.III on 18 July, for a total of five.
Harrison finally left the RAF, being transferred to the unemployed list on 14 June 1919.