After his wartime experiences, Nicol matriculated at Pembroke College, Cambridge, to read classics, graduating in 1949.
His thesis supervisor was Steven Runciman, with whom Nicol formed a lifelong friendship, nurtured in the Athenaeum Club.
He spent 1964–1966 as visiting fellow at Dumbarton Oaks, and was then Senior Lecturer and Reader in Byzantine History, University of Edinburgh (1966–1970).
In 1970 he was named to the historic chair of Koraës Professor of Modern Greek and Byzantine History, Language and Literature at the King's College London, a post he held until 1988.
He was the founding editor of Byzantine and Modern Greek Studies journal (1975),[5] whose publication he oversaw until 1983, and served as president of the Ecclesiastical History Society in 1975–1976.