He was the only son of Harold Macmillan, 1st Earl of Stockton, who was Prime Minister of the United Kingdom from 1957 to 1963.

Like his father, he was chairman of Macmillan Publishers, as well as a director of two news agencies.

Macmillan contested Seaham at the 1945 election, Lincoln in 1951 and Wakefield at a 1954 by-election.

They had four sons and a daughter: Macmillan was for a time the owner of Highgrove House, which he sold to the Prince of Wales in 1980.

He held the title for just 16 days, dying in Westminster, London, on 10 March 1984, following a heart operation.