From Wikipedia: 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 which aired from late 1969 to early 1974 on ABC. After its cancellation these Brady Bunch episodes were rebroadcast in syndication for decades as children’s programming, gathering a long lasting, cross-generational popularity which led to spinoffs and movies.