Plimpton's acting career spanned from the mid-1960s to the late-1980s. She created the role of "Crissy" in the original 1967 Off-Broadway production of Hair, and continued the role as a member of the original Broadway cast when the production moved to Broadway in 1968. In both productions, she sang the song "Frank Mills". Shelley Plimpton took a leave of absence from Hair to appear in Arlo Guthrie's film Alice's Restaurant, playing a 14-year-old who offers herself to Arlo, saying that she has already "made it" with several other Musicians and "you'll probably be an album some day." He gently rejects her advances, giving her his bandana as a souvenir and saying simply, "I just don't want to catch your cold." Plimpton also appeared in the 1969 Robert Downey, Sr., film Putney Swope as one half of an interracial college couple ("It started last weekend at the Yale-Howard game") in a satire of a pimple cream TV spot. She sings a duet, in which she concludes: "My man uses Face Off / He's really out of sight, and so are his pimples." In 1971, Plimpton appeared in Jim McBride's post-apocalyptic drama film, Glen and Randa, in which she portrays Randa, a young woman part of a group of scavengers who survived a nuclear apocalypse many years prior and sets off with her lover Glen (Steve Curry) to discover a ravaged world and to search for a city which Glen has seen in comic books. She worked with McBride once again when she was cast in the 1974 comedy film Hot Times. Her final film role was in the 1975 film Forplay.