Kerns got her start in show business as a dancer before turning to acting. She attended UCLA and majored in dance. There, she saw an advertisement for a Gene Kelly production called "Clown Around" and immediately jumped at the chance to audition. She joked in an interview that she sent the casting crew a high school senior picture of herself and all of her grades, hoping to get the part. To take the part, however, she would have to drop out of college and move to New York, so she did. After "Clown Around", Joanna also got parts in the New York Shakespeare Festival's production of "Two Gentlemen of Verona" and "Ulysses in Nighttown", where she was directed by Burgess Meredith.