She was the daughter of the banker and diplomat Andrew W. Mellon and Nora Mary (née McMullen) Mellon (1879–1937). Her parents divorced in 1912 and from 1921 to 1932, Ailsa served as her father's official hostess during his tenure as United States Secretary of the Treasury, and again when he was U.S. Ambassador to the United Kingdom in 1932–1933. Her only sibling was brother Paul Mellon (1907–1999), who was also a philanthropist and was known as a prominent owner/breeder of thoroughbred racehorses.[2]