He became famous in his own lifetime for embarking on an extra-marital relationship with his own half-sister, Mary Beauclerk (his mother's daughter by her second marriage), that produced four sons.

[5] St John entered the British House of Commons for Cricklade in 1782, at the age of 21, representing the constituency as Member of Parliament (MP) until 1784.

He persuaded her to marry him bigamously and then to live with him in obscurity first on the Continent, then in Britain, and finally in the United States.

Mary later (1797) married an Anglo-German Bavarian Count Franz von Jenison-Walworth, by whom she had legitimate issue and descendants.

[8] St John died, aged 63, at Pisa in Italy on a journey to regain his daughter's health.