Born in Llangynhafal, Denbighshire Wales in 1811, he graduated from Jesus College, Oxford in 1835 to become the Anglican curate of Llanfor, Merionethshire, where he married Elizabeth Lloyd Williams.
Nonetheless his enthusiasm and Welsh nationalist fervour, cause some to criticize him of being uncritical in his approach to the historical record and strongly influenced by Edward Williams (Iolo Morganwg, 1747–1826).
Williams was nevertheless considered for the chair of Celtic at Oxford University, and he was appointed by the government in 1858 to complete the editions of the medieval Welsh chronicles Annales Cambriae and Brut y Tywysogion, which had been left incomplete by Aneurin Owen (1792–1851), and which were published in 1860.
His collection includes letters to his family 1832–1898; page-proofs of parts of Brut y Tywysogion; drafts in manuscript of several of Ab Ithel's printed works, including Y Gododdin, The Traditionary Annals of the Cymry, Druidism, Brut y Tywysogion, and Annales Cambriae; contributions to the Cambrian Journal; his address at the Conwy Eisteddfod, 1861; papers relating to his collation to the rectory of Llanymawddwy, 1849; 'The Church of England independent of the Church of Rome in all ages'; sermons in English and Welsh; 'A few plain and practical Observations on Anglican Education'; the prize essay on the question 'whether the British Druids offered human sacrifices' by 'Hu Gadarn'; an essay entitled Traethawd ar Ddarganfyddiad yr America Gan Madog ab Owain Gwynedd, yn nghylch y ddeuddegfed ganrif' by 'Columbus'; an essay 'On the state of Agriculture and the progress of Arts & Manufactures in Britain, during the period & under the influence of the Druidical System' by Rev.
John Jones, Llanllyfni (see Archaeologia Cambrensis, Supplement 1850); 'Y Telynwr Cymreig, sef cyfarwyddiadau i ddysgu'r Delyn' by 'Dafydd Frenin'; four essays on 'Y manteision a ddeillia i'r Cymry o ddysgu yr iaith Seisnig' by 'Uthr Bendragon', 'Eiddil', 'Gwir Gymro', and 'Y cynyg cynta' 'rioed'; a 'History of Wales to the death of Llewelyn'; sections of a work on Christianity in Britain; letters addressed to Ab Ithel and also to his wife and daughters; press cuttings and extracts; sketches and medical and household recipes, and letters, 1884–1885, relating to the Ab Ithel Memorial Fund.