Prysor Williams was born Robert John Williams on 13 April 1891 in Trawsfynydd, Meirionnydd, in the heart of Gwynedd. His father Ellis, a carpenter, died when he was young, and his mother Eliza raised him and his sister with little more than parish support. He left Trawsfynydd British School at the age of ten to work as a farm labourer, later adopting the stage name Prysor after the river running through his home village.
When his mother remarried, the family moved south to the coalfields of Glamorgan, first to Abertridwr, where Williams began working underground at Senghennydd colliery, and later to the Abergorci pit at Treorchy in the Rhondda Valley. It was here, among the chapels and eisteddfodau of the mining valleys, that his lifelong Welsh connection deepened: he became a chapel precentor and organist, and threw himself into the amateur opera and drama societies of the coalfield.
At the 1928 National Eisteddfod in Treorchy, Williams met the BBC producer Daniel Haydn Davies and fellow collier-turned-actor David Moses Jones, contacts that changed the course of his life. In 1936 the novelist and radio producer Thomas Rowland Hughes invited him to appear in a radio play, and for the next thirty years Prysor Williams's voice was one of the most familiar in Welsh-language broadcasting, on both radio and the young medium of television. He also worked on stage, appearing at the Abbey Theatre in Dublin in J.O. Francis's Birds of a Feather and at the Globe Theatre in London in Jack Jones's Rhondda Roundabout, and took character parts in a handful of films, most notably Jill Craigie's Blue Scar (1949).
Williams married Margaret Mary Walters in 1917 and the couple had two daughters. He remained rooted in the Rhondda for the rest of his life and died at Treherbert on 13 October 1967, remembered by fellow professionals as one of the finest and best-loved character actors Welsh radio ever produced.