Ethel was a talented and prolific amateur artist in her own right who studied under Sir Arthur Cope at the Royal Academy Schools, winning the Gold Medal in 1909.
The Macmillan family returned from India to England in 1928 and lived at The Pound House in Edenbridge, Kent, his father joining a firm of civil engineers and working on the London drainage system south of the Thames.
He won school prizes in technical drawing and also excelled at mathematics, which he was taught by Edward Lockwood, a graduate of St John's College, Cambridge.
He regarded his biggest achievement at Swansea as influencing the development of the University as a whole, as well as steering the construction of the new engineering building, clearly visible on the skyline as the campus is viewed from the coast.
Macmillan's inaugural lecture was entitled The Communication of Ideas and dealt with the importance of arousing students' interest in engineering and conveying its significance more widely.
He continued writing and in 1962 published the book Non Linear Control Systems Analysis, also giving a paper on the topic to the annual conference of FISITA, the International Federation of Automotive Engineering Societies.
He served as a member of the Stability and Control Sub-committee which reported to the Mechanics Committee of the Royal Aeronautical Society at the time Concorde was being designed.
After eight years at Swansea, he accepted the post of Director of the Motor Industry Research Association (MIRA), moving there in 1964 to take over from Dr Albert Fogg.
Macmillan commissioned the construction of a linear induction motor for accelerating a vehicle quickly to a precise velocity in order to test how it would perform in a collision.
The design was based on a prototype developed by Professor Eric Laithwaite at Imperial College, London and the completed facility was opened by the Minister of Technology, Anthony Wedgwood Benn, in April 1968.
Changes to the funding regime were compounded by high levels of inflation during that period and, as Director, Macmillan faced challenging negotiations with the unions over pay and conditions.
Lockwood asked him for advice on various issues and Macmillan wrote two further chapters; the book Geometric Symmetry was published by Cambridge University Press in 1978 under joint names.
Another was part-written but its completion was delayed by the failing health of his wife, Anna, whom he had married in 1950 and whom he devotedly looked after in her last decade (she died in 2012) and it was never finished.