He succeeded his father to Blakesware and Gilston Park, Hertfordshire on 12 December 1767 and the Duke eventually agreed to release him from a commitment to stand at Lewes.
Beside possessing the most extensive property of any gentleman in this county, his additional estates in Essex, Middlesex, and Suffolk, make up a clear income of fifteen thousand pounds per annum ...
In his parliamentary character he has been uniform in his opposition to all the measures of the present Administration; and though no speaker, is one of the most constant attendants upon his legislative duty, and suffers no question to pass without that substantial indication of his political talents, a direct negative to the minister.
His activity out of the House is not less conspicuous than his honest zeal within it ... His character, as a private individual, is of that kind that totally exempts him from the smallest imputation of being under the direction of any improper motive in the enthusiasm of his public exertions.
Then he missed his participation in Parliament and at the age of 76, he accepted the seat of Higham Ferrers from his friend Earl Fitzwilliam and was returned in the 1812 general election.
[2] Plumer married as his first wife Frances Dorothy Carey daughter of Lucius Cary, 7th Viscount Falkland on 12 July 1760.