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; Sarah sought to marry one of her granddaughters to Frederick, Prince of Wales, and remained friendly with the prince and his family even after this plan had failed.
It has been alleged that the Duchess indulged in a lesbain affair with Queen Anne, although this has never been proven. In 2003, a Channel 4 documentary on British Television reignited these rumours.