He was educated at Harrow School and was admitted at Trinity College, Cambridge on 10 March 1829.
He lived at Melchbourne Park[2] and politically was a Tory, unlike his father.
[2] 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.
[1] He was succeeded in the title successively by his sons St Andrew and Beauchamp.