Born in Brooklyn, NY. A model at age four and a TV performer at seven, she burst into stardom as the muderous little brat of both the Broadway (1954) and screen (1956) versions of THE BAD SEED. She went on to play troubled teenagers in routine films of the 60s. On TV she appeared in "Mama: (1953-57) and "The Ropers" (1979-80) and briefly starred in her own series "Peck's Bad Girl" (1959). She also sang with rock bands. In 1988 she returned to movies after a long absence in a mother's role. Subsequent credits include DON QUIXOTE (1992), MOMMY (1995), MOMMY II: MOMMYS DAY (1997), THE SILENCING (2000), THE MEDICINE SHOW and CHOOSING MATTHIAS (both 2001), INHABITED, THE SILVERGLEAM WHISTLE and THE KISS (all 2003) and TARGET and SHALLOW GROUND (both 2004). McCormack has appeared frequently on television in series ("The Love Boat," "Fantasy Island," "Magnum P.I.," "Murder, She Wrote," "The Sopranos" and "ER") and in made-for-television movies.  Nominated for Supporting Actress 1956: THE BAD SEED 1 nomination