'Cwm Rhondda' icon faces ruin.
He was a leading figure in language and religion at that time, and his hymns are still sung today - not just in chapels but in sporting stadia all around the world.
After his death in 1791 aged 74, a chapel was built in his memory in his home town, Llandovery. Williams got his nickname 'Pantycelyn' from the farm where he was born, just outside the town.
But now the chapel's facade is crumbling away and pounds 25,000 is needed to match the funding from the local Townscape Heritage Initiative just to carry out the first phase of the work at the beginning of next year.
Built in 1886, it is described by the Evangelical Movement of Wales as 'the best memorial to William Williams'.
Chapel treasurer Margaret Kite said donations and that the were desperately needed. The chapel still attracts coachloads of visitors, some of whom attend services there on Sundays.
Ms Kite said, 'This chapel was built in his memory when the railway was being built through the town. Lots of English and Irish people moved in and the chapel is for English speakers in the area, so there are still people who speak English here who use it every single week.
'But the front is crumbling and desperately needs repairing so we hope people will be interested in offering some help.
'We have got money from the Townscape Heritage Fund which is working to regenerate Llandovery but we need pounds 25,000 of our own to start the work off. We will not get the extra funding until the work has been started.'
The chapel contains furniture donated by grateful Hindu converts from the Khasi hills of India who had been greatly moved by the power of his hymns.
The impressive pulpit of Caen stone has five panels: four depicting biblical scenes, while the fifth shows Williams writing on an ancient scroll.
It is surprising to find that the chapel is dedicated to an English language cause, for Williams normally preached in Welsh and most of his hymns and 90 or so prose publications are in the Welsh language.
But Ms Kite insists that Williams's influence on Welsh and other societies means the chapel is well worth saving.
Many famous people have been to pay their respects to Williams, and to many Welshmen he personifies their cultural heritage.: Ohio festival sings praises in Welsh:Hymn lovers in Ohio have been invited to the Salem Presbyterian Church of Venedocia, which is holding its annual Welsh Festival of Song, or Gymanfa Ganu, next week.
September 5 will see the American congregation singing the old Welsh hymns in four-part harmony under the direction of Geraint Wilkes.
'The tradition of the Gymanfa Ganu has been a part of the church history at Venedocia from early days and those who love to sing are invited to participate and enjoy,' said a spokesman for the church.
Mr Wilkes was born in Aberystwyth and has successfully combined a teaching and singing career. He has sung for Welsh, Scottish and Cornish events all over the US, Canada and Mexico.
The director has also taught Welsh and Celtic Studies throughout the state of Wisconsin for the university. When not involved in performing, he frequently leads tours of Wales.