Mary Trevor (1708-1780) was the daughter of John Morley Trevor (1681-1719) of Glynde, Sussex, and Lucy Montagu. She was well-connected on both sides of her family: her relatives included the Duchess of Marlborough, the Duke of Newcastle and the Earl of Halifax. She was a cousin of Lady Barbara Montagu, Sarah Scott’s companion. By 1742, she had become a close friend and companion of Lady Frances Williams, and her protector during her lengthy period of mental illness, preventing Lady Frances’ family and friends from visiting her. In 1777 she was attacked in the press by the Earl of Essex, husband of Lady Frances’ daughter Fanny, after members of the Essex family had attempted to gain access to Lady Frances to ask her to settle money on her granddaughter Elizabeth, who was engaged to be married; but Trevor was supported by Charlotte Walsingham, Lady Frances’ other daughter and guardian. She died on 28th March 1780, twenty-one months before Lady Frances. Eighty-two letters between Trevor and Walsingham survive in the Public Record Office of Northern Ireland.