Daughter of Algernon Seymour (1684-1750), 7th Duke of Somerset and Frances Thynne (1699-1754). Married (July 16, 1740) to Hugh Smithson, Baronet.
On the death of her father in 1750 she inherited the barony of Percy, and her husband became Earl of Northumberland. In 1766 they were elevated to Duke and Duchess of Northumberland. She was Lady of the Bedchamber to Queen Charlotte from 1761 to 1770 and was known as something of a socialite, often hosting parties at Syon Park, their London home.
She lays buried in Westminster Abbey's Northumberland Vault.
Boswell may have met Lady Northumberland during her tour of Scotland in August 1760, when she wrote in her journal for August 16 that she "[w]ent to Auchinlech the seat of Mr. Boswell, Lord Auchinlech". She found the location by the River Lugar romantic, but thought the house itself "terribly loaded with Ornaments of Trumpets and Maces and the Deuce knows what."1
On April 1, 1761, Boswell sent to her some verses written by him "when your Ladyship honoured Scotland with your Presence" in the previous summer. Boswell did not copy the actual verses into the transcription of his letter, but Hankins and Caudle, in The General Correspondence of James Boswell, 1757-1763, conjecture that they may have been 'Verses on Lord Warkworth's Going a Volunteer to Germany, 1760' which were later published in Donaldson's second Collection of Original Poems (1762).
During his stay in London 1762-1763, Boswell went to several of the famous parties at Northumberland House and initially believed himself to be a favourite of the Duchess'. On December 27, 1762, Boswell wrote a letter to the Duchess requesting her help to get him a position in the Guard. on December 13, 1762, he visited her and she told him, that she would recommend him to Lord Granby (1721-1770), Colonel of the Royal Horse Guards.
- 1Source: General Correspondence of James Boswell, 1757-1763, p. 54n4