Sarah Churchill, née Jennings, Duchess of Marlborough (May 29, 1660 - October 18, 1744), rose to be one of the most influential women in British history, largely as a result of her close friendship with Queen Anne. This came about some time after 1673, when she joined the household of the Duke of York (later to be King James II) and befriended his younger daughter; Anne was a few years Sarah's junior. They called one another by pet names, Sarah being "Mrs Freeman" and Anne "Mrs Morley". In 1677, Sarah married John Churchill, later to be created Duke of Marlborough, and they made a glittering couple. Churchill, though he had supported James, also had a role in bringing William of Orange to the British throne and was rewarded with an earldom.

Following Anne's accession to the throne, Sarah was made Mistress of the Robes and Keeper of the Privy Purse. She exercised great personal and political influence. However, the introduction into Anne's household of a politically aware rival, Sarah's own cousin, Abigail Hill (later Mrs Masham), the relationship between the queen and her old friend deteriorated greatly. In 1711, both Sarah and her husband fell out of royal favor. Anne's death in 1714 restored their fortunes, but the Duke died in 1722 and never saw the completion of Blenheim Palace, the house built for him by a grateful nation. It was left to Sarah to oversee the remaining work, and she was often in conflict with the architect, Sir John Vanbrugh.

Close links with the royal family remained. In a move which anticipated the life of Diana, Princess of Wales (1961-1997), Sarah sought to marry off her granddaughter Lady Diana Spencer (1710-1735), to Frederick, Prince of Wales. The Duchess remaining friendly with the prince and his family, however, even after this plan had failed.

It has been alleged that the Duchess indulged in a lesbian affair with Queen Anne, although this has never been proven. In 2003, a Channel 4 documentary on British Television reignited these rumours.

Sarah Churchill could also refer to Sarah Millicent Hermione Churchill, the daughter of Sir Winston Churchill and a descendant of the original Sarah.