Little is known of his subsequent career till 1695, when, in Dec. of that year, he was returned Member of Parliament for Pembroke borough; he held the seat till 1702. He re-entered Parliament and was Member for Haverfordwest till 1722. His father d. 18 Jan. 1696/7, and on 12 Dec. 1697, Sir John, as 4th baronet, m. Mary, daughter and heiress of Anthony Smith, a rich East India merchant. She d. 18 Nov. 1722, leaving three sons and three daughters. His sister Elizabeth's daughter m. Sir Robert Walpole in 1700.
From 1695 to 1737 Sir John was a leading figure in all the religious and philanthropic movements of the day — the Society for the Reformation of Manners, the S.P.C.K., the S.P.G., the East India Mission, and the Holy Club. He kept in constant touch with such religious reformers as A. H. Francke, A. W. Boehme, J. F. Osterwald, John and Charles Wesley, and George Whitefield, who was maintained by him for a while at Oxford. He was elected a member of the S.P.C.K. a month after it was founded, and remained its most influential member till his death. He made Pembrokeshire and Carmarthenshire the chief centres of the Society's work in Wales, founded twenty-two schools in the former county and several in the latter, and was chiefly responsible for the success of the early undertakings of his brother-in-law, Griffith Jones, Llanddowror (q.v.), husband of his sister Margaret.
Mrs. Mary Gwendoline Ellis, M.A., Aberystwyth