|A.K.A.||Lady Amelia Darcy|
|Countries||Kingdom of Great Britain|
|Birth||12 October 1754|
|Death||27 January 1784|
Amelia Osborne, Marchioness of Carmarthen, 12th Baroness Darcy de Knayth, 9th Baroness Conyers, 5th Countess of Mértola (née Darcy; 12 October 1754 – 27 January 1784), was a British peer and a Portuguese countess.
She was the only surviving child of Robert Darcy, 4th Earl of Holderness, and his wife, the former Mary Doublet. On 29 November 1773, she married Francis Osborne, Marquess of Carmarthen, in London, and they had three children:
- Lord George William Frederick Osborne (21 July 1775 – 10 July 1838), later 6th Duke of Leeds; married Lady Charlotte Townshend, daughter of the 1st Marquess Townshend, on 17 August 1797 and had issue.
- Lady Mary Henrietta Juliana Osborne (1776–1862); married Thomas Pelham, 2nd Earl of Chichester (28 April 1756 – 4 July 1826) in 1801 and had issue.
- Lord Francis Osborne (18 October 1777 – 15 February 1850), later 1st Baron Godolphin; married The Hon. Elizabeth Eden, third daughter of the 1st Baron Auckland, on 31 March 1800 and had issue.
The marchioness's portrait was painted in about 1764 by François Hubert Drouais.
On 16 May 1778, Amelia succeeded to the titles of 12th Baroness Darcy de Knayth and 9th Baroness Conyers in her own right, as the only surviving child of her father. Her right to the baronies of Darcy de Knayth and Conyers were eventually confirmed in 1798 (long after her death), and she also inherited the Portuguese countship of Mértola from him
Lord and Lady Carmarthen divorced in May 1779. It was said that the marchioness had become over-friendly with John "Mad Jack" Byron (father of the poet, Lord Byron), who visited her at home in Grosvenor Square when her husband was absent, and that they had been having an affair. Almost immediately after the divorce, she married Byron. They had one daughter, Augusta Leigh.
A year after the birth of her daughter by a year, in 1784, Amelia died and the titles were inherited by her eldest son, George.