 He succeeded his father as Principal Secretary of State (Ireland) in 1730, and on 6 May 1732 he was appointed to the Privy Council of Ireland.Southwell married on 21 August 1729 Catherine Watson (died April 1765), daughter of Edward Watson, Viscount Sondes and Lady Catherine Tufton and lived in Kings Weston House near Bristol.Their son Edward Southwell later became Baron de Clifford.