Nicholls worked at the Dublin Institute for Advanced Studies for a period during the 1960s.
He was a member of staff of the history department in University College Cork until his retirement in 2004.
[1] He came to national and international prominence as the author of Gaelic and Gaelicised Ireland in the Middle Ages, first published in 1972, and reprinted in 2003.
He is particularly regarded among his peers for his deep knowledge of late medieval and early modern historical sources in Irish, Latin, French and English languages.
Nicholls' areas of professional interests include: His work is honoured in the festschrift Regions and rulers in Ireland, 1100-1650: essays for Kenneth Nicholls (David Edwards, editor; Dublin, 2004), which is a collection of essays by several of the leading Irish historians of today.