Mary Duncan was born in Lutrellville, Virginia in 1895 and went to Hollywood after critics praised her acting on Broadway. There is not a lot of readily available information about Mary, but I found this little bit of trivia in New Movie Magazine, September 1931, from the Hollywood Who's Who section - "Mary Duncan had just settled herself in Paris, France, for a holiday, when she received a cable from Howard Hughes to return at once for a role in a new picture. So home she came." Research tells me the film was The Age for Love, starring Billie Dove. It was the tenth of her sixteen films.
Mary had only one lead role, in Murnau's
City Girl, produced in 1930 and considered to be one of his best films. Her last appearance was in
Morning Glory, where Katherine Hepburn won her first Oscar.
While making her eleventh film, Five and Ten (1931), she became friends with the film's lead, Marion Davies. They attended a polo match, where Davies introduced Mary to Stephen "Laddie" Sanford. And a couple of years later, Time Magazine, Monday September 11, 1933, reported "Married. Stephen ("Laddie") Sanford, 34, carpet tycoon, international poloist; and Film Actress Mary Duncan, 28; in Manhattan."
After her marriage, Mary retired from films and moved with him to Palm Beach, Florida. For many years the former Mary Duncan reigned as the grande dame of Palm Beach society. She continued her charity work until very late in life, and in 1993 died quietly in her sleep at the age of 98.

Mary Duncan - What do you think? Allure?