Frances Stewart, Marchioness of Londonderry (née Pratt; 15 April 1751 – 18 January 1833), was an English aristocrat and mistress of a large landed and politically connected household in late Georgian Ireland.

Correspondence with her stepson, Robert Stewart, Viscount Castlereagh (British Foreign Secretary at the Congress of Vienna), and with the English peer and politician John Petty, record major political and social developments of her era.

In 1770, King George III had demanded and secured his dismissal as Lord High Chancellor of Great Britain for his openly expressed sympathies with John Wilkes and the American colonists.

[3][4] It was a quality admired by her county neighbour, herself the centre of a circle of politically engaged women: Elizabeth Rawdon, the dowager Countess of Moira had, like Lady Frances's father, been open in her sympathy for the American cause.

During the United Irish risings in the early summer of 1798 Camden was Lord Lieutenant of Ireland and was served, as Chief Secretary, by Lady Frances's stepson, Robert Stewart, Viscount Castlereagh.

She was a friend of Jane Greg, reputedly "the head of the [United Irish] Female Societies" in Belfast,[6] and in the view of General Lake responsible for "very great mischief" in that disaffected town.

[20] But It is also possible that Londonderry, aware that his wife had continued to send for Porter's offending paper, the Northern Star,[21] and had corresponded with Greg, believed the minister to have been an original source of her wayward, and potentially compromising, political sympathies.

[24] Petty's correspondence with Lady Frances, which he maintained until his death in 1809, reveals that she continued to entertain criticism of government policy in Ireland, including the Act of Union that her step son helped push though the Irish Parliament in 1800; of the Anglican church establishment and the tithes it levied atop rack rents; of “British tyranny in navigation”, and of religion ("a bad substitute for common sense").

[3] While continuing to take a keen interest in political affairs and corresponding regularly with Castlereagh throughout his war-time service as War, and subsequently as Foreign, Secretary, Lady Frances also immersed herself in local projects.