In 1761 Southwell successfully contested Bridgwater on the interest of his distant cousin Lord Egmont. He appears in Henry Fox’s list of Members favourable to the peace preliminaries, December 1762, and voted with the Administration on the Address on the preliminaries,
Early in October 1763 Southwell declared himself a candidate for Gloucestershire. He was supported there by Norborne Berkeley, trustee for the young Duke of Beaufort, and Member for the county. Berkeley obtained for Southwell the support of George Grenville who arranged for him to vacate the seat at Bridgwater, and ensured his unopposed return for Gloucestershire.
He died 1 Nov. 1777.
