Morrill began his academic career with a number of short term appointments. For the 1970/71 academic year, he was Keasbey Lecturer in history at
Trinity College, Oxford. He held a
junior research fellowship at Trinity from 1971 to 1974. He was also a college
lecturer in history at
St Catherine's College, Oxford, for the 1973/74 academic year. Then, from 1974 to 1975, he was a lecturer in modern history at the
University of Stirling. He was elected a
Fellow of the British Academy (FBA) in 1995, and served as Vice-President in 2001–09. He was Chair of the Research Committee of the AHRB (2002–05), and also served as a Vice-President of
Royal Historical Society. Morrill was President for 10 years of the
Cromwell Association, "a body that seeks to promote public knowledge about and interest in
Cromwell and his age". According to the online
Bibliography of British and Irish History, he has published (up to July 2016) 116 books, essays and articles but some of his major contributions have been in developing online datasets – as General Editor of the
Royal Historical Society Bibliography of British and Irish History and of the British Overseas (1992–99) — now the online Bibliography of British and Irish History, as Chair of the Management Committee of the project that put 8,000 survivor statements from the
1641 'massacres' in Ireland, and as General Editor of an imminent (5 volume and online) edition of all the recorded words of Oliver Cromwell. On 6 July 2009, Morrill delivered his lecture 'The British Revolution in the English Provinces, 1640-9' as part of
The Marc Fitch Lectures. == Church career ==