He married firstly in 1913 Jean Sutherland, daughter of the Reverend John Sutherland of Uitenhague, South Africa, and, secondly, Mona Constance Mary Tweedie, daughter of Admiral Sir Hugh Tweedie, KCB, and his wife, Constance Marion (Mona) Crossman. He acknowledged the help of his first wife in the production of his first four books and of his second wife in the production of his later works. He met Mona Tweedie in 1931 and was married to her after divorce from his first wife in 1936. Mona Macmillan became an author in her own right, publishing
Introducing East Africa in 1952,
The Land of Look Behind: A Study of Jamaica (1957),
Mediator and Moderator: The life of Sir Henry Barkly (1969),
Champion of Africa: W.M. Macmillan, the second phase (1985), and a memoir, ''Mona's Story'', posthumously, in 2008. She also edited the writings of the controversial Zambian archbishop, Emmanuel Milingo, and, with her daughter, Catriona, the correspondence of the Pratt family. She died in Oxfordshire in 2003 at the age of 95. There were no children from the first marriage and four children from the second marriage. His elder son, the eminent art historian and writer
Duncan Macmillan FRSA FRSE HRSA, is emeritus Professor of Art History at the
University of Edinburgh and was curator of the university's
Talbot Rice Gallery. He is the author of books including
Scottish Art, 1460-2000,
Painting in Scotland: The Golden Age,
Painting in Scotland in the Twentieth Century and ''Scotland's Shrine: The Scottish National War Memorial''. He is the art critic of
The Scotsman. His younger son, Hugh Macmillan, has worked at universities in Swaziland, Zambia and South Africa, and is currently a research associate at the African Studies Centre, University of Oxford. He is the author of
The Lusaka Years: The ANC in Exile in Zambia, 1963–94 (2013);
An African Trading Empire: The story of Susman Brothers & Wulfsohn (2005), and of
Zion in Africa: The Jews of Zambia (1999, with Frank Shapiro). 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. Macmillan, historian and social critic (1989). His elder daughter, Lindsay, married Alexander Arthur Dow, and had a career as a teacher in Zambia and Scotland. 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. She edited, with her mother,
Exiles of Empire, the nineteenth century correspondence of the Pratt family between Scotland, India and Australia. ==Honours==