William Miller Macmillan was born in 1885 in Aberdeen, the youngest son of the Rev. John Macmillan, a missionary academic in India and South Africa, and his wife Elizabeth Caird Lindsay. He spent much of his early life in South Africa, moving to Cape Colony with his family in 1891 to join his father who was working at Victoria College, Stellenbosch. He was educated at Stellenbosch Boys’ High School and then at Victoria College, Stellenbosch (later the University of Stellenbosch) for two years. In 1903 he became one of the first Rhodes Scholars, taking a place at Merton College, Oxford, to study modern history. On graduating he enrolled in a course in divinity at the United Free Church College in Aberdeen. He also studied divinity in Glasgow, and in 1910 attended the Friedrich Wilhelm University in Berlin for a semester.

In 1911 Macmillan took position as lecturer in history and economics at Rhodes University College, Grahamstown. While there he took a particular interest in poverty among white South Africans, in urban and rural areas. This research eventually developed into his book The South African Agrarian Problem (1919). In 1917 he became the first professor of history at the Johannesburg School of Mines (later the University of the Witwatersrand).

In 1920 Macmillan was given access by the Philip family to the papers of the London Missionary Society superintendent in South Africa, Dr John Philip. His study of these papers led him into an analysis of the history of race relations and colonial policy in South Africa. Books written by him on the subject include The Cape Colour Question (1927), Bantu, Boer and Briton (1929) and Complex South Africa (1930). The Philip papers were destroyed in a fire at the Witwatersrand University library in 1931. Macmillan deposited his invaluable notes on this archive in Rhodes House Library in 1959 (MSS. Afr. s. 216-219). On finishing his work on Dr John Philip, Macmillan travelled extensively in Central, East and West Africa and the West Indies, looking into issues of colonial governance. Findings from these travels resulted in books including Africa Emergent (1938, republished 1949) and Warning from the West Indies (1938).

Macmillan’s anti-segregationist views brought him into conflict with the South African government, and he left the University of the Witwatersrand on sabbatical in 1932. He did not return to the university, resigning his chair in 1933. He returned to England at the low point of the depression. He was unable to find an academic post but he became an active member of various pressure groups including the Anti-Slavery Society and the Labour Party Committee on Imperial Questions. He served on the Colonial Office Advisory Committee on Education from 1940-1943 and worked as Director of Empire Intelligence for the BBC from 1941-1943. In the latter year he was appointed Senior Representative of the British Council in West Africa, based in the Gold Coast (now Ghana), a post he held until 1946.

Macmillan had been selected as Labour candidate for Inverness but was unable to stand in the 1945 general election due to his absence in West Africa. In 1947 he was appointed Director of Colonial Studies at the University of St. Andrews, a post he held until his retirement in 1954. He held a position as Visiting Professor at the University College of the West Indies from 1954-1955.

He was married twice, first in 1913 to Jean Sutherland, and in 1936 to Mona Tweedie, with whom he had four children. He died in 1974 at his home in Long Wittenham, Oxfordshire.

Mona Constance Mary Macmillan (née Tweedie) was born in 1908, the daughter of Admiral Sir Hugh Tweedie and his wife Constance Marion Crossman. She was educated at home until 1922, when she started school at the Convent of Sacred Heart in Roehampton.

In 1931, Mona travelled with her parents and younger sisters to South Africa, where her father was Commander-in-Chief Africa Station, based at Simon's Town from 1931-1933. On the boat she met William Macmillan, whom she would marry in 1936 following his divorce from his first wife. While in South Africa she took a course in history at the University of Cape Town, and on her return to England she did a secretarial course in London and attended seminars in social anthropology given by Professor Bronislaw Malinowski at the London School of Economics in 1932-1933.

Mona Macmillan was an author in her own right, publishing works including Introducing East Africa (1952), The Land of Look Behind: A Study of Jamaica (1957), Sir Henry Barkly: Mediator and Moderator, 1815-48 (1969), Champion of Africa: W.M. Macmillan, the second phase (1985), and a memoir, Mona's Story (published posthumously 2008), and editing the writings of Archbishop Emmanuel Milingo. She gave courses and lectures on Africa and its history to the Workers’ Educational Association. She was an active member of the Women’s Institute and of the Associated Country Women of the World. She died in 2003 at Rush Court, a nursing home in Shillingford, Oxfordshire.