Phillips's first film role came in Dennis Hopper's film "The Last Movie" (1971), in a minor part; she and Hopper married shortly after the production on October 31, 1970, but the union lasted only eight days. Two years later, she was cast in a lead role in the thriller film "Dillinger" (1973) as John Dillinger's girlfriend, Billie Frechette. Phillips claimed she got cast by pretending to be half Cherokee, like her character. The film was critically acclaimed, and "Variety" said of her performance: "Phillips, making her film bow after having been a member of the Mamas & the Papas singing group, scores heavily as Dillinger's girlfriend", while the "New York Times" noted it as "mildly effective". Phillips was nominated for a Golden Globe Award for Most Promising Newcomer for her performance. Reflecting on the film, Phillips said: "I was so lucky to have been surrounded by really great actors. Everybody in that movie was a real actor: Warren Oates, Ben Johnson, Cloris Leachman, Richard Dreyfuss, Harry Dean Stanton. It was just a wonderful, wonderful experience for me and I had so much support and so much help and so much encouragement. That was really my first movie. Dennis' movie ["The Last Movie"] was a lot of improvisation and craziness." Phillips remained a lifelong friend of co-star Stanton.