A graduate of Hollywood High who grew up in Long Beach and the San Fernando Valley, Kellerman took acting classes with the likes of Jack Nicholson, Robert Towne, Carol Eastman, and Shirley Knight before landing acting roles on TV shows, including an appearance in the "Star Trek" pilot, and in "The Many Loves of Dobie Gillis" (playing a girl who is captured by the musky smell of Maynard's hair tonic).
Struggling with her weight, Kellerman used diet pills starting at the age of 18 (dexamyl spansules, which "made me feel like Felix the Cat on the inside. They burnt out my nervous system and eventually became a popular street drug.") Of her iconic shower scene in M*A*S*H, she wrote, "For someone like me, who had ridden the roller-coaster of pregnant women's urine, dexamyl capsules, apple diets, and fruit fasts to always run back to cookies, brownies, and candy—the mainstays of my diet—getting naked was not at the top of any 'to-do' list." She recalled of her time working on the ensemble-style film with director and Very Important Pothead Robert Altman that, "After shooting, we'd smoke a little grass and some would have a drink. We'd all go to the dailies together."