[1][2] Originally from Canada, Macmillan received a Bachelor of Science degree in engineering physics in 1950 from Queen's University.
He later attended the University of Cambridge from 1952 to 1955, where he was a member of Gonville and Caius College, and earned a Master of Science degree.
By the early 1960s he had left the aeronautical industry and moved to Ontario where he began working in the pulp and paper sector.
Upon learning this, Macmillan contacted Dame Ann Dowling, chair of fluid mechanics at the University of Cambridge.
In consideration of Macmillan's then-advanced age, special dispensation was granted to conduct his viva voce via telephone.