Virginia-born Mary Duncan went to Hollywood after critics praised her acting in the lead in "The Shanghai Gesture" on Broadway. While making Five and Ten (1931), she became friends with the film's lead, Marion Davies. The two women attended a polo match, where Davies introduced Duncan to Stephen "Laddie" Sanford, an international polo star and director of the Bigelow-Sanford Carpet Company. She and Sanford married in 1933, after which she retired from films and moved with him to Palm Beach, Florida. They also maintained three homes in the New York area.

In retirement, she devoted herself to philanthropic works, doing in major fund-raising for several charities. In addition, the Sanfords were socially very prominent, and for many years the former Mary Duncan reigned as the grande dame of Palm Beach society. A neighbor who became a close friend was Rose Kennedy, mother of President John F. Kennedy. Those whom the Sanfords entertained at their mansion included the King and Queen of Jordan and the Duke and Duchess of Windsor. She continued her charity work until very late in life, and died quietly in her sleep, aged 98.