Gwynne-Jones was born in Richmond, Surrey. He was educated at Bedales School and then qualified as a solicitor, but never practised. He instead developed a love of art and began painting watercolours. In 1914 he began a course at the Slade School of Fine Art, but three months later was commissioned into the Cheshire Regiment. He was wounded and awarded the Distinguished Service Order (DSO) at the Battle of the Somme in 1916. He returned to the Slade after demobilisation in 1919 and in 1923 became Professor of Painting at the Royal College of Art. He remained at the Royal College for the remainder of his career, and also gained renown for his own painting, most notably portraits and paintings of flowers.

Gwynne-Jones became a Royal Academician in 1956. He was rather belatedly appointed Commander of the Order of the British Empire (CBE) in 1980, only two years before his death.