Armitage was commissioned into the Royal Artillery as a second lieutenant on 6 January 1900, as the army expanded due to the ongoing Second Boer War in South Africa, where he was sent. He was promoted to lieutenant on 3 April 1901, while still in South Africa. After the end of this war in June 1902, Armitage was attached to the 74th battalion Royal Field Artillery, which left Durban for British India in October 1902, and was stationed at Lucknow, Bengal Presidency. He later fought in the World War I, serving in France and Belgium. He was appointed Chief Gunnery Instructor at the School of Artillery in 1925, Commandant of the Royal School of Artillery in 1927 and commander of the 7th Infantry Brigade in 1929. He went on to be Commandant of the Staff College, Camberley in 1934, General officer commanding (GOC) of the 1st Infantry Division. The division was sent to Palestine during the 1936–1939 Arab revolt. He was aided throughout this difficult period by his General Staff Officer Grade 1 (GSO1), Thomas Hutton. After handing over command of the division to Major General The Honourable Harold Alexander in early 1938, Armitage was Master General of the Ordnance in India in 1938, in which role he served in World War II before retiring in 1942.