Returned on Cumberland’s interest at Windsor in 1754, Fitzwilliam invariably followed his lead in politics. On 28 Feb. 1757 he questioned Admiral Keppel about Admiral Byng’s court martial—the only time he is known to have spoken in the House.1Walpole, Mems. Geo. II, ii. 347. His only recorded votes were with Opposition over the peace preliminaries, 9 Dec. 1762, when the message from the Duke ordering him not to vote did not reach him in time;2Harris’s ‘Debates’. over Wilkes on 15 Nov. 1763, and over general warrants, 18 Feb. 1764. In July 1765 Rockingham classed him as ‘pro’, but in November 1766, after Cumberland’s death, as ‘Swiss’ (ready to support any Administration), while in March 1767 Newcastle described him as ‘doubtful or absent’.
He did not stand at the general election of 1768.
He died 31 July 1789.