Macmillan married the Honourable
Katharine Ormsby-Gore, daughter of
William Ormsby-Gore, 4th Baron Harlech, on 22 August 1942. They had four sons and a daughter: •
Alexander Daniel Alan Macmillan, 2nd Earl of Stockton (born 10 October 1943) • Joshua Edward Andrew Macmillan (1945–1965) • Hon. Adam Julian Robert Macmillan (1948–2016) • Hon. Rachel Mary Georgia Macmillan (1955–1987) • Hon. David Maurice Benjamin Macmillan (born 1957); married English fashion designer
Arabella Pollen in 1995 and has issue. Macmillan was for a time the owner of
Highgrove House, which he sold to the
Prince of Wales in 1980. Upon his father's elevation to the peerage as
Earl of Stockton on 24 February 1984, Macmillan acquired the
courtesy title Viscount Macmillan of Ovenden. He held the title for just days, because he died on 10 March 1984, in
Westminster, London, following a heart operation. He was 63. His father outlived him by almost three years, dying in December 1986 at the age of 92. Macmillan's son
Alexander has held the title 2nd Earl of Stockton since the death of the first Earl. ==Arms==