Professor MacMillan is a Visiting Professor at the Blavatnik School of Government since 2021. She was the fifth Warden of St Antony’s College from 2007 to 2017 and prior to that Provost of Trinity College, Oxford and professor of History at the University of Toronto. From 1975 until 2002 she was a member of the History Department at Ryerson University in Toronto, where she also served as the Chair of the Department.

She is a Fellow of the Royal Society of Literature and sits on several not-for-profit boards and the editorial boards of International History and First World War Studies. She is an Honorary Fellow of St Hilda’s College, Oxford and has honorary degrees from the University of King’s College, the Royal Military College, Ryerson University, Toronto, the University of Western Ontario, Huron University College of the University of Western Ontario and the University of Calgary. 

In 2006 Professor MacMillan was invested as an Officer of the Order of Canada, and in 2016 she was appointed as a Companion of the Order of Canada. In the 2018 Queen’s New Year’s Honours List, Professor MacMillan was appointed a Companion of Honour (CH) for services to higher education, history and international affairs.

Professor MacMillan was elected an Honorary Fellow of Lady Margaret Hall, University of Oxford, in December 2017. She has a long-standing relationship with St Antony's College, Oxford, to which she returned as a Senior Associate Member in 1993 and was elected to an Honorary Fellowship in 2003.