While a pupil at Chigwell School, Essex, Williams taught himself Cornish and became a bard of the Cornish Gorseth while still in his teens, taking the bardic name Golvan ('Sparrow'). He read classical languages, English language and Celtic in Oxford. After short periods in the universities of Belfast (where he received his PhD) and Liverpool, he was appointed lecturer in Irish in University College Dublin in 1977. In 2006, he was appointed Associate Professor in Celtic Languages there. He married Patricia Smyth from Portadown, County Armagh in 1976. They have three children, Benedict, Jerome, and Dominica who are bilingual in English and Irish.

Williams has written widely on the Celtic languages and their literatures. His works on Irish include the editions The Poems of Giolla Brighde Mac Con Midhe (1980) and Pairlement Chloinne Tomáis (1981); I bPrionta i Leabhar (1986), an account of Protestant writing in Irish during the seventeenth century; Díolaim Luibheanna (1993) a discussion of Irish plant names and plant lore; and Armas, a handbook of Irish heraldry in Irish, which he illustrated himself. He also was joint editor of Stair na Gaeilge (‘the History of Irish’, Maynooth, 1994) for which he contributed chapters on the Irish dialects of Leinster and on Manx.

As P. Berresford Ellis points out, Williams was the first professional Celtic scholar to study revived Cornish in depth.P. Berresford Ellis, The Cornish language and its literature, London and Boston: Routledge & Kegan Paul, 1974 pp.188-190 In 1990 Williams published an article "A problem in Cornish phonology", demonstrating that the phonemes represented by the graphemes /tj/ and /dj/ had never been part of the language and should therefore be removed from Kernewek Kemmyn. He continued his critique of this variety of Cornish in Cornish Today (Kernewek Dre Lyther 1995) in which he also set out his own emended Unified Cornish (Unified Cornish Revised or UCR). Williams elaborated UCR in Clappya Kernowek (Agan Tavas, 1997) and in his English-Cornish Dictionary (Agan Tavas, 2000). Spyrys a Gernow published his Testament Noweth, the first complete Cornish translation of the New Testament from the original Greek in 2002. He gave the O’Donnell lectures in Oxford in May 1998, when he spoke on consecutive days on Manx and then Cornish. This was the first time that an O’Donnell lecture had ever been devoted to the Cornish language.

Among important articles by Williams Cornish one might include the following: "'Linguistically sound principles': the case against Kernewek Kemmyn", Cornish Studies, 4, (1997); "Pre-occlusion in Cornish", Studia Celtica 32 (1998); "Indirect Statement in Cornish and Breton", Cornish Studies 6, (1998); "Saint in Cornish", Cornish Studies 7 (1999) and the review, "'A modern and scholarly Cornish-English dictionary': Ken George’s Gerlyver Kernewek Kemmyn (1993)", Cornish Studies, 9 (2001). Williams together with Graham Thomas edited the Middle Cornish play Bewnans Ke, which had been donated to the National Library of Wales, Aberystwyth, in 2000. Their edition was published by Exeter University Press in association with the National Library of Wales in October 2006.

Williams was awarded first prize in the Gorsedd literary competition three times in the early 1960s. More recently he won first prize for Cornish poetry in the Cornish Gorsedd in 1997, 1998 and 1999. In 1974 Berresford Ellis wrote, "Probably the most able young writer in the language today is N.J.A. Williams (Golvan), a worthy successor to Edwin Chirgwin." Some of Williams’s poetry in Cornish was published by Tim Saunders in The Wheel (1990) and Nothing Broken (2006). The Welsh critic Bobi Jones says in the introduction to this anthology, "Nicholas Williams, the well-known scholar, is also the T. Gwynn Jones of Cornwall -- polished, classical, rather conservative, soundly rooted in medieval romanticism." Williams’s "Ancow Arthur", a translation of Tennyson’s "Morte D'Arthur", published in Delyow Derow 15 (1996) is an example of his verse.

Nicholas Williams is a Fellow of the Linnaean Society of London and was awarded honorary membership of the Irish Translators' and Interpreters' Association for his Cornish New Testament. He has translated four books in the series the Letts Pocket Guides into Irish, Mammals, Insects, Medicinal Plants and Edible Plants. His translation of Alice's Adventures in Wonderland into Irish (2003) received excellent reviews"Níos aistíche agus níos aistíche" in Lá, 24 November 2003; copy of the review here. In November 2004, he published his Irish translation of Through the Looking-Glass. He is currently working on a translation of The Hobbit by J. R. R. Tolkien.

's coat of arms was granted by the Chief Herald of Ireland on 1 November 2006. The escutcheon's formal blazon is in Irish, translated here as Argent two piles throughout gules three cinquefoils counterchanged ('On white, two red triangles throughout the shield, three cinquefoils in the reverse colour'); the colours are those of the City of London, and the piles form a W. Along with this was granted a crest, On a wreath of the colours a pied wagtail bearing in its beak ragged robin all proper ('On a red and white wreath, a pied wagtail bearing in its beak ragged robin, all in their natural colours'). "Willie wagtail" is common name used for this bird in Northern Ireland.. The motto is Grásta Dé a bhfuil agamsa "The Grace of God is all I have"...a pun, where a bhfuil agamsa plays on Williams.