He was prominent in the foundation of the International Eisteddfod at Llangollen, Wales in 1947 and become its first musical director.
He started playing the piano and violin when he was very young and began composing in his teens.
In 1937 he founded the Gwynn Publishing Company with the aim to increase the repertoire of traditionally inspired vocal and choral music.
Among the many collections of folk-songs and music published by Gwynn Williams are: Old Welsh Folk-songs (1927); Caneuon Traddodiadol y Cymry (Vol 1: 1961, Vol 2: 1963); Welsh National Music and Dance (four editions: 1932 to 1971) and Harp Tunes of Wales (1962).
His compositions include: Tosturi Duw (God's Mercy) (1943), My Little Welsh Home, Breuddwyd Glyndwr (Glendower's Dream) (1950), Dawns y Medelwyr (Harvester's Dance) (1940) and Welsh Playbook (1925).