He represented the Cornish constituencies of Bossiney (1786–90), Tregony (1790–95) and St Germans (1806–12) in the British Parliament and succeeded his brother as 4th Baron Rokeby in 1829.
Robert Harley, 1st Earl of Oxford and Earl Mortimer (1661–1724), Member of Parliament for Radnor and Tregony
This recommended Robert Harley to the notice of the Boscawen family, and led to his election, in April 1689, as the parliamentary representative of Tregony, a borough under their control, whilst at the same time acting as High Sheriff of Herefordshire.
Sir Thomas Trevor, 1st Baronet (c. 1612-1676), his son, Member of Parliament for Monmouth and Tregony