He was the son of the Reverend John Macmillan (born Glen Urquhart, Inverness, Scotland 1831, died Stellenbosch, South Africa, 1909).
His mother was Elizabeth Caird Lindsay (born Glasgow, Scotland 1845, died Cape Town, South Africa, 1927).
He attended the Boys High School, Stellenbosch, and did the first two years of the BA degree at the Victoria College, matriculating in 1901 and passing the intermediate exams in 1903.
Macmillan joined the Fabian Society in 1911 and remained an evolutionary socialist or social democrat for most of his life, reverting to liberalism in old age.
He was the only English-speaking delegate at the Dutch Reformed Church congress on White Poverty, which was held at Cradock, Eastern Cape in 1916.
In 1918, Macmillan did substantial fieldwork on white poverty in rural areas, resulting in lectures and a book, The South African Agrarian Problem and its Historical Development, published in Johannesburg in 1919.
In the following decade, Macmillan produced two important books, The Cape Colour Question (1927) and Bantu, Boer and Briton: The Making of the South African Native Problem.
Following his return to the United Kingdom in 1932, Macmillan was unable to secure academic employment and remained self-employed until after the outbreak of World War II.
He was, however, active in a number of pressure groups including the Friends of Africa, the Anti-Slavery Society, the British Labour Party's committee on Imperial Questions, and with the Trade Union Congress.
[2] He was based at Accra in the Gold Coast (now Ghana), but was also responsible for the inauguration of the council's work in Nigeria, Sierra Leone and the Gambia, including libraries and scholarship schemes.
After the war, the postwar Labour government supported Macmillan's appointment as director of Colonial Studies at the University of St Andrews in Scotland.
Macmillan retired from St Andrews in 1954 and served for one year, 1954-5, as acting professor of History at the University of the West Indies in Jamaica.
He has also written pocket biographies of Chris Hani, Jack Simons and Oliver Tambo and is the co-editor with Shula Marks of Africa and Empire: W.M.
His younger daughter, Catriona, married Lieutenant Colonel Alistair Miller OBE, Queen's Royal Irish Hussars, and subsequently had a career as a hotelier in Dorset.