In 1614 St John was elected Member of Parliament for Bedford and was re-elected in 1621 and 1624. In 1626 and 1628 he was elected MP for Barnstaple. He sat until 1629 when King Charles decided to rule without parliament for eleven years.[5]
St John married Margaret Draynor, the widow of Thomas Draynor and daughter of John Trye, of Hardwick, Gloucestershire. He survived her death in 1656 and caused a white marble monument, adorned with pilasters, entablature, pediment, and two Cupids, to be erected in her memory in the church of St Leonards, Shoreditch.[6] He left no children.