On 17 Oct 1652 Henry St John 1st Viscount St John was baptised.

In Oct 1679 Henry St John 1st Viscount St John (age 26) was elected MP Wootton Bassett which seat he held until March 1681. He also held the seat in 1685–87, 1689–95 and 1698–1700.

Evelyn's Diary. 20 Dec 1684. A villainous murder was perpetrated by Mr. St. John (age 32), eldest son to [his father] S' Walter St. John (age 62), a worthy gentleman, on a knight of quality [Sir William Estcourt], in a tavern. The offender was sentenc'd and repriev'd. So many horrid murders and duels were committed about this time as were never before heard of in England, which gave much cause of complaint and murmurings.

Note. Following the aquittal of Edward Nosworthy the jury repaired to the Globe tavern in Fleet Street. An altercation broke out between St John and Francis Stonehouse. St. John and Edmund Webb both ran Estcourt through with their swords. Both were found guilty of murder, and condemned to death. St. John's mother [[his mother] Johanna St John Baroness St John Lydiard Tregoze (age 53)] obtained a pardon for him at the reported price of £16,000. St. John was expected to go abroad for some time, but publicly entered the next parliamentary election at Wootton Bassett a few weeks after the close of his case, and was duly elected to James II’s first Parliament.

In 1685 Henry St John 1st Viscount St John (age 32) was elected MP Wiltshire which seat he held until 1698.