St. John was named after his ancestor Margaret Beauchamp, heiress of the Bletsoe estate. As the youngest of six surviving sons, he received only £50 in his father’s will, but the family’s local prestige secured a match which brought him an estate of 450 acres.
At the general election of 1624 St. John was supplanted as knight of the shire by his eldest brother’s heir, Oliver St. John II*. Despite his family’s extensive parliamentary patronage, St. John did not find another seat until 1626, when his brother Sir Alexander left a vacancy at Bedford by migrating to Barnstaple. In this Parliament St. John was included on committees for bills concerning clerical pluralism (14 Feb.), malting (9 Mar.) and the partition of the newly drained marshes of Canvey Island, Essex (28 March).
St. John and his eldest brother, now earl of Bolingbroke, were among the magistrates who failed to persuade the Bedfordshire subsidymen to contribute to a Benevolence demanded by the king shortly after the acrimonious dissolution of the 1626 Parliament.
St. John was restored to the commission of the peace in December 1628, and stayed aloof from the political controversies of the Personal Rule, even though his relative Oliver St. John† was John Hampden’s* counsel in the Ship Money case. This may have been partly due to his entanglement in the financial problems of his friend Sir Thomas Cheyney of Sundon, Bedfordshire and his nephew Lord St. John (Oliver St. John II), whose creditors were still pursuing him in the early 1650s.
Reinstated to the Bedfordshire bench at the Restoration, St. John sold a reversion of his estates to his nephew Sir Oliver St. John of Woodford in 1661, and probably spent his last years living with Lord St. John’s widow at Welby, Leicestershire. In his will of March 1667 he left the latter two gold portingales, and bequeathed her family most of the £2,000 he raised from the sale of Tilbrook. He confirmed this will on 8 July 1667 and was dead by 23 Aug., when it was proved at Leicester.
