John Armstrong was born in Hastings, Sussex. He studied at St John's College, Oxford (1912-1913) and at the St John's Wood School of Art, London (1913-1914)
His first one-man exhibition was held at the Leicester Galleries, London in 1928 and he became a member of Unit One in 1933 after which his work acquired a surrealist characteristic. His inclusion within this group alongside Ben Nicholson, Henry Moore and Barbara Hepworth represented not only a confirmation of his essential understanding of the values of abstract art but also his success at rendering them in paint. Armstrong had a considerable influence on the British Surrealist painters, who studied his evocative dreamscapes and metaphoric works of the 1930's.
In 1938 the Tate Gallery bought their first picture of his, 'Dreaming Head', from his exhibition at the Lefevre Gallery. After the war his work was represented in many exhibitions including those at the Whitechapel Art Gallery, the Festival of Britain and 'Art in Britain 1930-40', which focused on Axis, Circle and Unit One at Marlborough Fine Art. He was commissioned as an Official war artist during World War II. In the post war years he exhibited widely and was elected an Associate of the Royal Academy in 1966.