His maternal grandparents were Robert Greville, 4th Baron Brooke (son of Robert Greville, 2nd Baron Brooke) and the former Anne Dodington (daughter and heiress of John Doddington of Breamore, MP for Lymington).

[1] Montagu was returned as a Whig MP for Huntingdonshire at the 1734 British general election.

He vacated his seat when he succeeded his brother, who died without issue, to the peerage and the Manchester Duchy in 1739.

She was a sister-in-law of Hugh Boscawen, 1st Viscount Falmouth and niece of John Churchill, 1st Duke of Marlborough.

Together, Harriet and Robert were the parents of:[1][5] The 3rd Duke of Manchester died on 10 May 1762 leaving four children.