[2] While a student at Carmarthen Grammar School in 1743, Williams was converted after hearing a sermon by George Whitefield.

[2] He was ordained a deacon in 1745 and briefly held curacies in the diocese of St Davids, at Eglwys Gymyn, Swansea, and at Llangrannog and Llandysilio Gogo in Ceredigion.

[2] He was dismissed from his curacies and refused ordination as a priest due to his Methodist sympathies and became associated with Water Street Chapel Carmarthen.

[2][3] In 1770 Williams began to publish copies of Welsh language bibles at an affordable price, with commentaries on each chapter.

The commentaries assisted in avoiding the claim to a monopoly over the right to publish the Welsh Bible which was asserted by the Royal Printer and the Universities of Oxford and Cambridge.

During Peter Williams' own life, the Welsh Calvinistic Methodists remained within the Established Church.

He founded and supervised for 14 years Lampeter grammar school, which prepared young men for ordination in the Established Church.