McIntyre was a teaching fellow at Washington University from 1955 to 1956. After completing his PhD, in 1959 he became an assistant lecturer, and later lecturer, in Commonwealth and American history at the University of Nottingham. In 1966, he was appointed a professor in history at the University of Canterbury, in Christchurch, New Zealand, where he remained for the rest of his career. He retired in 1997, and was awarded the title of professor emeritus. He continued to write and research. An expert on the constitutional and military histories of the Commonwealth of Nations and British Empire, McIntyre published and advised governments. He served as consultant to the Committee on Commonwealth Membership, and compiled its report which was accepted by Heads of Government at Kampala in 2007.Report of Committee on Commonwealth Membership, London, Commonwealth Secretariat, Sept 2007
