An article by Florabel Muir in the New York Daily News in 1931 began: "Mary Duncan was in Hollywood nearly all of last year looking for work with little or no luck.

On September 1, 1933, Duncan married Stephen "Laddie" Sanford,[7] who was an international polo player as well as director of the Bigelow-Sanford Carpet Company, after which she retired from films.

She spent much of her remaining years working with several major charities, and earned a reputation as a socialite in Palm Beach, Florida.

[8] She kept herself active by playing golf twice a week and swimming every morning before breakfast, which helped her maintain her size 8 figure.

Duncan was the last known person to have in her possession a copy of the lost Murnau film 4 Devils; Martin Koerber, curator of Deutsche Kinemathek, has speculated that her heirs may still have the valuable print somewhere.