Mary was the eldest of three daughters and heiress of Andrew Fountaine [1808-1874] by his wife, Caroline, whom he married in 1848. On 3 February 1876, Mary wed her cousin, Algernon Charles Fountaine [1851-1909], who then succeeded Mary in Andrew Fountaine's Narford Hall estate. The couple had a son, Charles Andrew (later Vice-Admiral), who succeeded his father in the Narford estate, and a daughter, Marion.