Haynes joined his son-in-law John William Egerton as Member for Brackley on the interest of the latter’s cousin, the 3rd Duke of Bridgwater. Nothing was expected of him except a general support of Pitt’s administration. He was listed as an opponent of the repeal of the Test Act in Scotland in 1791. Evidence of his attendance is scant: he was absent with leave from ‘severe indisposition’, 2 June 1795, and a defaulter on 3 Apr. 1797.
He died 18 June 1811, leaving his estate to his wife with remainder to his daughter Charlotte Catherine Ann, since 1803 Countess of Bridgwater.