Plimpton was born in Roseburg, Oregon, to a father who ran an auto parts store. She is a "very distant" cousin of writer George Plimpton. She moved to New York with her researcher mother when she was 14, after her father died. Her acting career spanned from the mid-1960s to the mid-1970s. 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."