McCormack was born Patricia Ellen Russo in Brooklyn, New York, the daughter of Elizabeth (née McCormack), a professional roller skater, and Frank Russo, a fireman. She attended New Utrecht H.S. She is an aunt of fellow actor and New York City lawmaker Alfred Cerullo.
She was a child model at the age of four and began appearing on television at the age of seven. She made her motion picture debut in Two Gals and a Guy (1951) and appeared in the television series Mama from 1953 to 1956.
Her Broadway debut was in Touchstone (1953), and following this, she portrayed Rhoda Penmark, an eight-year-old sociopath and fledgling serial killer, in The Bad Seed (1954). She was nominated for an Academy Award for Best Supporting Actress for her role in the film version, The Bad Seed(1956). In 1957, she was cast by Orson Welles in his film of Don Quixote, but filming had to be abandoned for budgetary reasons, and was never fully completed. When a version was edited together in 1992, some years after Welles's death, it did not include any of McCormack's scenes even though they had been central to the framing of the plot.
McCormack briefly starred in her own series, Peck's Bad Girl in 1959.
She married restaurateur Bob Catania in 1967, and they had two children before their marriage was dissolved. After a half-dozen teen roles during the 1960s, her film career declined but she continued to work in television. In 1970 she played Linda Warren on the soap opera The Best of Everything.She guest starred on The Streets of San Francisco, Season 2, episode Blockade. She also portrayed a San Francisco paramedic on the season 7 1 1/2 hour Emergency! films What's a Nice Girl Like You Doing...? and The Convention. She resumed her cinema career with Bug in 1975.
She continued to act in secondary roles, mostly in television with occasional film appearances. She had recurring roles in popular television series, including The Ropers, Dallas, Murder, She Wrote and The Sopranos. When Kathryn Hays left the CBS soap opera As the World Turns for an extended period of time, McCormack took Hays' role until she returned.
In 2008, McCormack played First Lady Pat Nixon in the feature film Frost/Nixon.
Her most recent film appearance was in the Hallmark Channel presentation of Citizen Jane (2009) starring Ally Sheedy, Sean Patrick Flannery andMeat Loaf. Her most recent television appearance is a starring role of "Connie Campolitarro" in the first season of the hit web series "Have You Met Miss Jones" (2012) which can be seen on BlipTv.com.
In addition to her Oscar nomination, McCormack also received a Golden Globe nomination for The Bad Seed. In 1956, she received the Milky Way "Gold Star Award" as the most outstanding juvenile performer.
Her star on the Hollywood Walk of Fame is at 6312 Hollywood Boulevard.
McCormack was a guest for the 2009 Mid-Atlantic Nostalgia Convention where her films were screened, and she received an award during the closing ceremonies.