Born in Manhattan and raised in Queens by a mother who was once a professional roller skater, McCormack was modeling at age four and by age six was acting, playing the daughter of a singing couple in the feature film "Two Gals and a Guy" (1951). She joined the cast of the successful family comedy/drama "Mama" (CBS, 1953-56), as a newcomer to the Norwegian-immigrant family. Also in 1953, she made her Broadway debut in the short-running "Touchstone". The following year, she first created the role of Rhoda in the stage version of "The Bad Seed". With the success of the 1956 feature, which earned her a Best Supporting Actress Oscar nomination, McCormack landed roles as part of a pioneer family undergoing hardships in Wisconsin in "All Mine to Give" (1957) and a temperamental child star in the "Kathy O'" (1958). She was back on the small screen as a 12-year girl who can't decide whether she can still be a tomboy or if it is time to become a young lady on "Peck's Bad Girl" (CBS). Throughout her teen years, the actress alternated between features (like 1960's "The Adventures of Huckleberry Finn") and television (e.g., "Young Dr. Malone" NBC, 1962-63).
As with most child stars, roles became more scarce as McCormack matured. In the 60s, now an ingenue, she found roles in low-budget forgettables for American International before turning to the small screen. By 1970, she was again on a soap opera, ABC's "The Best of Everything", during its short life, and later played Kim Reynolds on "As the World Turns" (CBS, 1975-76). McCormack returned to primetime as the friend of social climber Helen Roper (Audra Lindley) on the ABC sitcom "The Ropers" (1979-80) and had a recurring role on "Dallas" (CBS, 1981-82). McCormack made her TV-movie debut in 1983 with "Night Partners" (CBS) and co-starred as Paul LeMat's wife in the NBC miniseries "On Wings of Eagles" (1986). She continued doing episodic guest appearances into the 90s. In 1995, McCormack starred as the murderous, malicious "Mommy", a thriller made directly for home video sale. The character might have been a grown-up Rhoda Penmark, had she not been struck dead by a bolt of lightning.