Mordaunt was married on 7 December 1866 to Harriet Moncreiffe (7 February 1848 – 9 May 1906), daughter of a Scottish baronet. Mordaunt was a stolid country squire with no interests beyond hunting and shooting, and his wife was a giddy young beauty. She had already caught the eye of the Prince of Wales (the future King Edward VII). Lady Mordaunt had licence, or thought she did, to carry on affairs with other men. While Mordaunt hunted, fished, or sat in Parliament, she entertained numerous lovers, including the Prince and several of his aristocratic friends.