 Macmillan married Lady Dorothy Cavendish, the daughter of the 9th Duke of Devonshire, on 21 April 1920.Her great-uncle was Spencer Cavendish, 8th Duke of Devonshire, who was leader of the Liberal Party in the 1870s, and a close colleague of William Ewart Gladstone, Joseph Chamberlain and Lord Salisbury.Lady Dorothy was also descended from William Cavendish, 4th Duke of Devonshire, who served as Prime Minister from 1756 to 1757 in communion with Newcastle and Pitt the Elder.