Despite a brief spell at St John’s Wood School of Art in London, Armstrong was largely self-taught as an artist. Following service during World War I he became friends with the actress Elsa Lanchester who ran a late-night theatre club. This led to his involvement in stage and costume design. As a painter he developed an interest in Surrealism and was a member of the English avant-garde group, Unit One. His painting was subsequently greatly affected by his experiences as an official war artist from 1940-5, both in style and subject. Although Armstrong became increasing disabled by Parkinson’s disease in his later years, he continued to paint and travel. At seventy-three he was elected an Associate member of the Royal Academy.