Born in Teddington, Middlesex in 1920, Dunstan was educated at St. Paul's School in West London, where it soon became apparent that his interest lay in the art room rather than in academic studies. From school he went on to study briefly at the Byam Shaw School of Painting and Drawing (1939) before taking up a scholarship at the Slade (1939-41) in Oxford, where it had relocated to at the start of the war. It was here he first met fellow student Diana Armfield, whom he was to later marry in 1949.
Influenced by Bonnard, Sickert and Vuillard, Dunstan established himself with his intimate figures in interiors and views of Venice, which he exhibited annually at the Royal Academy Summer exhibition and N.E.A.C. exhibitions. From 1952 onwards he started exhibiting regularly in London, firstly with Roland, Browse and Delbanco in Cork Street, then later with Agnews, and in 1981 he began exhibiting with the Stremmel Gallery in Reno, Nevada, USA.
Dunstan taught at various prestigious schools including the West of England School of Art (1946-49), Camberwell School of Art (1950-64), Byam Shaw (1953-74), Ravensbourne Art College (1959-64) and City & Guilds of London (1964-69).
He was elected a member of the New English Art Club in 1946 and served as president of the Royal West of England Academy from 1979 -84. In 1968 he was made a full member of the Royal Academy.
He wrote six books including 'Learning to Paint' and 'Painting Methods of Impressionists' and his works are represented in the National Portrait Gallery, the Royal Collection, the Tate Gallery and the Museum of London.