St. John was called to the bar in 1784, but he apparently never practised. He was a trustee of the estate of his young kinsman Lord Clinton, patron of Callington, which provided him with a seat there on a vacancy in 1803. St. John silently supported Addington’s and Pitt’s administrations: he was at first listed ‘doubtful’, then twice as a supporter, 1804-5. He voted against Whitbread’s censure motion on Melville, 8 Apr. 1805.
On 2 Nov. 1805 he wrote to Pitt asking to become deputy commissary general of musters—describing himself as a ‘zealous supporter of your measures’. He received, according to the endorsement, ‘a civil refusal’. On 8 Dec. 1805 he wrote again, this time applying to become deputy paymaster of Nova Scotia and referring to his ‘domestic calamities’ (his wife died in June 1805 leaving him with seven children to bring up).
On the advent of the Grenville administration in March 1806 Lord Clinton wrote to Lord Grenville that he expected St. John ‘to relinquish his seat’ and added that he was anxious to bring in a friend of theirs.
