Mary Osborne, Duchess of Leeds (1723 - 3 August 1764) (née Lady Mary Godolphin), daughter of Henrietta Godolphin, 2nd Duchess of Marlborough and Francis Godolphin, 2nd Earl of Godolphin. She was a granddaughter of the powerful government trio during the reign of Queen Anne: the famous general and politician, John Churchill, 1st Duke of Marlborough, and his wife Sarah, Duchess of Marlborough, through her mother; and Sidney Godolphin, 1st Earl of Godolphin through her father.
Lady Mary was considered to be the illegitimate child of the famous playwright, William Congreve, who was having a long-term relationship with Henrietta, Duchess of Marlborough. She was, however, recognised by Francis as his own daughter, and was raised by him with the other Godolphin children. Despite this, gossip spread rapidly about Mary's true father, and her grandmother Sarah, Dowager Duchess of Marlborough did not recognise her until 1740, when the two reconciled their differences. It was at this time that Sarah arranged her marriage, to Thomas Osborne, 4th Duke of Leeds, and the two were married on 26 June 1740. They had one child, Francis Godolphin Osborne (styled Marquess of Carmarthen, who was later 5th Duke of Leeds and Secretary of State for Foreign Affairs), who was born on 29 January 1751.
Mary died in 1764, aged 41.