During the course of his research, he became interested in the relationship between republicanism and empire in the works of John Milton and was increasingly attracted to the discipline of intellectual history.
Funded by a Harkness Fellowship, he took two years off from his PhD to retrain as a historian at Princeton's Institute for Advanced Study.
[1] He was awarded his doctorate in history from Cambridge in 1992 with his dissertation The British empire and the civic tradition, 1656–1742, a study of the relationship between English literature and Britain's imperial ventures in the Americas.
[2][3] After completing his PhD, Armitage remained at Cambridge until 1993 as a junior research fellow at Emmanuel College.
In 2008 Harvard named him a Walter Channing Cabot Fellow for "achievements and scholarly eminence in the fields of literature, history or art".