St John was the son of Oliver St John (of Cayshoe) and his wife Sarah Bulkeley, daughter of Edward Bulkeley of
Odell, Bedfordshire and sister of
Peter Bulkeley. St John's sister, Elizabeth St John (1605-1677), married Revernd Samuel Whiting (1597-1679) and emigrated to the
Massachusetts Bay Colony in
Boston in 1636. St John's uncle, Reverend
Peter Bulkeley (1583-1689), was also an influential Puritan who left England to live in Massachusetts Bay Colony having fallen out in 1633 with
King Charles I, primarily over the King's reissuing of the
Declaration of Sports. St John was one of the leaders of the Parliamentary opposition to
King Charles I of England. St John was at Cambridge University with his brother-in-law the Rev. Samuel Whiting, matriculating from
Queens' College, Cambridge at Lent 1616. He was then admitted at
Lincoln's Inn on 22 April 1619. He was called to the bar in 1626. St John appears to have got into trouble with the court in connection with a seditious publication, and to have associated himself with the future popular leaders
John Pym and
Lord Saye. In 1638 he defended
John Hampden, along with co-counsel
Robert Holborne, on his refusal to pay
Ship Money, on which occasion he made a notable speech which established him as a leading advocate. In the same year, he married as his second wife, Elizabeth Cromwell, a first cousin of
Oliver Cromwell, to whom his first wife also had been distantly related. The marriage led to an intimate friendship with Cromwell. == Political career ==