St. John was a younger son of a large, wealthy, and influential family.
St. John does not seem to have stood again, though he remained active in local affairs. As a j.p. he was unenthusiastic about enforcing the Benevolence of 1626, and returned a vague account of its reception in the eastern division of Northamptonshire ‘lest by descending too far into particulars something should be fastened upon which may produce an unexpected prejudice’. It is therefore unsurprising that his name was omitted from the Forced Loan commission the following year.
