Maureen Denise McCormick was born on August 5, 1956, in Woodland Hills, California, the youngest of four children to Richard McCormick, a teacher, and Irene McCormick, a housewife, with three older brothers named Michael, Dennis, and Kevin.

She began her career at age six by winning the Baby Miss San Fernando Valley beauty pageant, which led to television commercials for products like Barbie dolls and Kool-Aid, followed by early guest spots on shows such as Bewitched, My Three Sons, and Camp Runamuck by age seven.

Her feature film debut came in 1969 as a student in Elia Kazan's The Arrangement, but she achieved lasting fame that same year at age twelve when cast as the eldest daughter, Marcia Brady, on the ABC sitcom The Brady Bunch, portraying the perky, popular teenager in a blended family from 1969 to 1974.​

Maureen McCormick reprised her role as Marcia in numerous spin-offs, including the animated The Brady Kids, the variety series The Brady Bunch Hour in 1976-1977, the telefilm The Brady Girls Get Married and its short-lived sitcom The Brady Brides in 1981, and the 1988 reunion movie A Very Brady Christmas, while also sharing a sporadic on-set romance with co-star Barry Williams during the original run.

After the show's end, she struggled with typecasting, cocaine and quaalude addiction, bulimia, and depression in her twenties, which derailed her career, including flubbing a Raiders of the Lost Ark audition with Steven Spielberg and trading sex for drugs, though she made guest appearances on series like Happy Days, The Love Boat, Vega$, and Fantasy Island, plus films such as Skatetown, U.S.A. (1979), A Vacation in Hell (1979), and The Idolmaker (1980).​

In the 1980s and 1990s, she performed on stage as Wendy Darling in a 1983 touring production of Peter Pan and made her Broadway debut as Betty Rizzo in Grease in 1994, while also starring as country singer Barbara Mandrell in the 1997 TV biopic Get to the Heart: The Barbara Mandrell Story.

She portrayed Rebecca Crane on the soap opera Passions in 2000 and appeared on reality shows like Celebrity Fit Club, Gone Country—which spawned her short-lived spin-off Outsiders Inn—and the Australian I'm a Celebrity... Get Me Out of Here!, alongside roles in films like Texas Lightning (1981), Return to Horror High (1987), and the 1995 Brady Bunch movies.

Maureen McCormick detailed her battles with addiction and recovery in her 2008 New York Times bestselling memoir Here's the Story: Surviving Marcia Brady and Finding My True Voice, marking a turning point toward sobriety and renewed career stability.​

Married to actor and director Michael Cummings since 1985, with whom she has a daughter named Natalie born in 1989, McCormick has continued selective work into the 2020s, including voice roles in animated projects, theater, and Brady-related reunions, while advocating for Special Olympics as an ambassador and pursuing interests in writing, designing, and country music.