Returned for Peterborough on the Fitzwilliam interest after coming of age, Fitzwilliam was warm in support of Sir Robert Walpole during the last days of his Administration, weeping on his resignation. He was on the court list for the secret committee set up by the Commons to inquire into Walpole’s Administration,
On his marriage he distinguished himself not only by the magnificence of his apparel and the ‘new equipage ... made in the chinese fashion being all over japanned’ in which he took his bride to be presented at court, but also by making himself very ill by a surfeit of small beer and ices.
On 2 July 1746 he successfully applied through Newcastle for an English earldom, with the support of Lord Rockingham.
Lord Fitzwilliam has grumbled more than once to me being slighted notwithstanding what has been done for him when so young, and without any parliamentary merit, and not a very sure interest in this borough.
He died 10 Aug. 1756.