Mary Howard, Duchess of Norfolk and 7th Baroness Mordaunt (c.1659–17 November 1705) was a British peer.

Born Lady Mary Mordaunt, she was the only surviving child and heiress of Henry Mordaunt, 2nd Earl of Peterborough and Lady Penelope O'Brien, daughter of Barnabas O'Brien, 6th Earl of Thomond.

On 8 August 1677, she married Henry Howard, Earl of Arundel, who later succeeded his father as Duke of Norfolk in 1684.

[citation needed] The duke and duchess were divorced in 1700,[1] due to her adulterous relationship with Sir John Germain, 1st Baronet, whom she married a year later; a previous effort by the Duke to divorce her in 1692 had failed.

This no doubt reflects their opinion of the Duke's own conduct: his own counsel remarked that his charges against Germain were an example of "the pot calling the kettle black".