William St John inherited both the barony and baronetcy on the death of his young nephew, Sir Paulet St Andrew St John, 8th Baron St John, in May 1714. Scion of an ancient family, through his father St John was related to the Tudors, while through his mother’s family he traced his line to a brother of King Stephen. St John’s branch of the family had been based at Woodford in Northamptonshire since the early seventeenth century, the manor house being assessed in 1662 at a comparatively modest 16 hearths (Bletsoe castle and Melchbourne were twice as large).
St John was introduced into the House in the midst of the first session of the 1713 Parliament on 27 May 1714. A letter of 12 May stated that he had made his first appearance on that day, but this is presumably a mistake.
St John took his seat in the new Parliament on 17 Mar. 1715; his subsequent career will be examined in the next volume of this work. He died on 11 Oct. 1720 and was buried ten days later at Bletso, being succeeded by his next brother, Rowland St John†, as 10th Baron St John.
