 St John married Eleanor Hussey, daughter of Vice-Admiral Sir Richard Hussey Hussey on 12 March 1838.In addition to his sons St Andrew and Beauchamp, who succeeded successively to the barony, he had a third son, Edmund, and two daughters, Eleanor and Laura (12 June 1842 – 21 October 1919), who married Connolly McCausland of Drenagh (13 May 1828 – 25 June 1902) on 8 June 1867, and were the great-great grandparents of comedian, actor and TV presenter Alexander Armstrong.St John died aged 62 at Melchbourne, and was buried at Bletsoe.