In June 1762, it became known that Lady Mary was suffering from cancer. Even though she tried to conceal her illness from her family as long as possible, she grew rapidly weaker that month. She wrote her last letter but with difficulty on 2 July to Lady Frances Steuart; in this letter, she wrote "I have been ill a long time, and am now so bad I am little capable of writing, but I would not pass in your opinion as either stupid or ungrateful. My heart is always warm in your service, and I am always told your affairs shall be taken care of." Lady Frances's son was in London, and when he visited Lady Mary's house, he was summoned to her bedroom where he was surrounded by her relatives, including Lord and Lady Bute. Lady Mary then ordered her relatives to leave the room, saying of Lady Frances's son, "My dear young friend has come to see me before I die, and I desire to be left alone with him." She died on 21 August 1762 at her house in Great George Street, and was buried in Grosvenor Chapel the day after she died.