She studied dance at the Ruth St. Denis dancing school. There she was noticed by movie director D. W. Griffith who used her as a dancer in his movie "Intolerance" (1916). Griffith then gave her a role in "A Romance of Happy Valley" (1919). The movie featured the popular acting team of Robert Herron and Lillian Gish. Dempster's first starring role came in the movie "The Girl Who Stayed at Home" (1919). Lillian Gish was supposed to get the role, but was reportedly "tired" and Griffith cast Dempster instead. Critics gave Dempster only mild praise. Her first solo starring role was in "The Love Flower" (1920). It was at this time critics began comparing her to Lillian Gish. But it was Griffith who directed her to sound like Gish and his other legendary female stars. It was about this time that Dempster became romantically involved with Griffith. Her first non-Griffith movie was in 1922. She starred in "Sherlock Holmes" with John Barrymore. Her movie "Isn't Life Wonderful?" (1924) was well received by critics. She starred in two movies with W. C. Fields for Paramount Pictures, "Sally of the Sawdust" (1925), and "That Royle Girl" (1925). Her best performance may have been in her last movie "The Sorrows of Satan" (1926).