| Constituency | Dates |
|---|---|
| Derby | 30 June 1746 – 1754 |
| Saltash | 1754 – Nov. 1756 |
| Harwich | 13 Dec. 1756 – 4 July 1758 |
M.P. [I] 1725 – 58.
P.C. [I] 19 Nov. 1741; sec. to the ld. lt. [I] 1741 – 44; ld. of Admiralty June 1746 – Nov. 1756; ld. of Treasury Nov. 1756 – June 1759; jt. postmaster gen. June 1759 – Nov. 1762; P.C. [GB] 12 July 1765; jt. postmaster gen. July 1765 – Nov. 1766.
Sui juris in Ireland, in England Duncannon moved in the orbit of the Cavendishes, ranked as an Old Whig, and regularly voted with the Government. As a lord of the Admiralty he was returned in 1754 for the Admiralty borough of Saltash which he vacated in November 1756 on being made a lord of the Treasury: the Duke of Devonshire as its first lord, finding himself in a difficulty with regard to the choice of candidate for Harwich, where a vacancy had occurred, most ingeniously resolved it by returning his own brother-in-law, Duncannon.
Having succeeded his father in the peerage, Bessborough resigned office on the dismissal of the Duke of Devonshire in October 1762; was reinstated as postmaster general by Rockingham; and at first remained in office under Chatham. When Chatham insisted on giving Lord Edgcumbe’s place of treasurer of the Household to John Shelley, Bessborough offered his own place to accommodate Edgcumbe, but the offer was not accepted, and Bessborough, together with other Rockinghams, withdrew from the Government in November 1766. Henceforth he was in steady opposition. In 1773 he was one of the five peers who protested, under Rockingham’s lead, against the proposed Irish absentee land tax. By 1782 he had lost interest in politics, and was too old to take office.
He died 11 Mar. 1793.