She received critical acclaim for the role on Broadway and was nominated for an Oscar for Best Supporting Actress for her performance in Mervyn LeRoy's film adaptation.

[1] Her acting career has continued with both starring and supporting roles in film and television, including Helen Keller in the original Playhouse 90 production of The Miracle Worker, Jeffrey Tambor's wife Anne Brookes on the ABC sitcom The Ropers, Adriana La Cerva's mother in The Sopranos, and as Pat Nixon in Frost/Nixon (2008).

[6] McCormack made her motion-picture debut in Two Gals and a Guy (1951) and appeared as Ingeborg in the television series Mama with Peggy Wood from 1953 to 1956.

Her Broadway debut was in Touchstone (1953), and the following year, she originated the role of Rhoda Penmark, an 8-year-old psychopathic serial killer, in Maxwell Anderson's The Bad Seed (1954)[7] with Nancy Kelly.

She portrayed Helen Keller in the original 1957 Playhouse 90 production of William Gibson's The Miracle Worker opposite Teresa Wright.

McCormack briefly starred in her own series, Peck's Bad Girl, with Marsha Hunt and Wendell Corey in 1959, and had a leading role in MGM's remake of The Adventures of Huckleberry Finn with Eddie Hodges.

In the early 1960s, she starred in a series of popular teenage delinquent films, including The Explosive Generation with William Shatner and The Young Runaways.

[12] On March 20, 1956, she received the Milky Way "Gold Star Award" as the most outstanding juvenile performer, in which Sal Mineo was placed third and Tommy Rettig second.