Armstrong was born on 11 June 1897 and was educated at Cheltenham College. After attending the Royal Military Academy Sandhurst, he was commissioned into the East Surrey Regiment in April 1915. Seconded to Machine Gun Corps, he served in France from 1916 to 1918, where he was twice wounded and, in March 1918, made a prisoner of war. Repatriated in December 1918, he served in North Russia in 1919 and was awarded the Military Cross for gallantry. He was present in Mesopotamia 1920-21 and on the Indian North West Frontier 1930-31, serving much of his inter-war service with the 1st Battalion, East Surrey Regiment in Egypt, Sudan, Hong Kong and India.