McCormick was born in Encino, California, the daughter of Irene (née Beckman) and William McCormick, a teacher. At the age of six she won the Baby Miss San Fernando Valley beauty pageant. McCormick first appeared on national US television in 1964 on Barbie doll and Chatty Cathy commercials. She lent her voice to a redesigned Chatty Cathy doll in 1970. Through the later 1960s McCormick appeared in two episodes of Bewitched and played guest roles on I Dream of Jeannie, Honey West, The Farmer's Daughter and My Three Sons.

McCormick played the eldest daughter, Marcia, a popular girl with a perky personality in The Brady Bunch, an American television sitcom about a blended family that aired from late 1969 to early 1974 on ABC. After its cancellation Brady Bunch episodes were rebroadcast in syndication for decades as children's programming, gathering a long lasting, cross-generational popularity that led to spinoffs and movies.

After The Brady Bunch McCormick made guest appearances on many television series such as Happy Days, Donny & Marie, Love Boat, Streets of San Francisco and Fantasy Island, along with supporting roles in a few B-movies such as 1979's Vacation in Hell, Skatetown, U.S.A. and 1987's Return to Horror High. However, perhaps owing to the ongoing broadcast of Brady Bunch reruns in syndication, she may have become typecast. McCormick later claimed she failed to a get a part as a prostitute or heroin dealer for the movie Midnight Express because of this. McCormick was the first actress to play Rebecca Crane on the soap opera Passions but she was not put on contract.

In 2007 McCormick was a cast member on the fifth season of VH1's reality show Celebrity Fit Club, hoping to lose 30 pounds she had gained since her mother died of cancer and needing to move her disabled brother into an assisted living facility. McCormick lost 34 pounds and, in June of that year, was the individual winner of the series. In 2008 she became a spokesperson for Children International. Later that year, she became a cast member on the CMT reality show Gone Country. This led to a spin-off reality series called Outsiders Inn in which she opened a bed and breakfast inn at Newport, Tennessee. On March 15, 2009 McCormick appeared on Comedy Central's roast of Larry the Cable Guy.

She also appeared in an episode of Scrubs as JD's dream wife; however, she is married to him as Marcia Brady.

McCormick and fellow Brady Bunch cast member Barry Williams had a sporadic romance, their first romantic kiss happening in the summer of 1972 while filming the Hawaiian episodes.

In her autobiography, McCormick wrote that her grandmother died from syphilis in a mental institution, having gotten the disease from her husband who had caught it in Europe at the time of World War I (he committed suicide a week after his wife's death). McCormick's mother had contracted syphilis in utero and McCormick dealt with a life-long but unfounded fear she would also get the disease. Hence, she later said her favorite scenes in The Brady Bunch were those that called for her to cry, since this allowed her to show feelings she drew from those fears.

McCormick met Michael Jackson socially in 1973.

Following the cancellation of The Brady Bunch McCormick went through years of addiction to cocaine and Quaaludes that harmed her career. McCormick later claimed she sometimes traded sex for cocaine. She flubbed an audition with Steven Spielberg for a part in Raiders of the Lost Ark, showing up high on cocaine and sleepless for three days. She lost her reputation for reliability in Hollywood, and one producer threatened that she would never work as an actress again. She had two unwanted pregnancies, both of which ended in abortion. She also dealt with bouts of depression and bulimia.

On March 16, 1985 she married Michael Cummings, who had never seen or heard of The Brady Bunch. They fell in love upon meeting in a church. McCormick and Cummings have one child, daughter Natalie Michelle, born May 19, 1989. The family lives in Westlake Village, northwest of Los Angeles.