It was created in 1763 for Robert Knight, 1st Baron Luxborough (1702–1772), Member of Parliament for Great Grimsby, Castle Rising and Milborne Port.
The 1st Earl was the only son of Robert Knight, Cashier of the South Sea Company, by his first wife.
Henrietta was thus half-sister of Henry St John, 1st Viscount Bolingbroke, the son of her father's 1st marriage.
On 8 August 1745 he was created Baron Luxborough, of Shannon, in the peerage of Ireland.
On 14 May 1763 was further raised in the Irish peerage by Bute's successor George Grenville, Prime Minister between 1763–65, being created Viscount Barrells and Earl of Catherlough.