Kyffin Williams studied at the Slade School of Fine Art, London from 1941 to 1944. He went on to become the Senior Art Master at Highgate School from 1944 to 1973, and in 1968 gained a Winston Churchill Fellowship to record the Welsh in Patagonia.
Kyffin Williams's first one-man exhibition was held at P & D Colnaghi, London in 1949. Subsequent solo exhibitions were held at the Leicester Galleries, London, Glynn Vivian Museum & Art Gallery, Swansea, Howard Roberts Gallery, Cardiff and the Tegfryn Gallery, Menai Bridge. Kyffin Williams has exhibited at the Thackeray Gallery, London biennially from 1975 and also exhibits regularly at the Albany Gallery, Cardiff. A retrospective of his work was held in 1987 at the National Museum of Wales, Cardiff, and subsequently toured to the Mostyn Gallery, Llandudno and the Glynn Vivian Art Gallery, Swansea. A portraits retrospective was held at Oriel Ynys Mon, Llangefni in 1993.
Kyffin Williams was President of the Royal Cambrian Academy from 1969 to 1976, and again from 1992. He was made an Honorary Fellow of University College, Swansea (1989), University College, Bangor (1991) and University College of Wales, Aberystwyth (1992). In 1991 he received the Medal of the Honourable Society of Cymmrodorion. Kyffin Williams lived and worked in Gwynedd, Wales.