Johnson's career began at age six, when she played the role of Chrissy Seaver on the sitcom Growing Pains from 1990 to 1992. The character's age was accelerated from a toddler between seasons for plotting purposes. By the time she was 12, Johnson had been in the casts of eight TV series. She later reprised her role as Chrissy in The Growing Pains Movie and Growing Pains: Return of the Seavers. In the one season series Phenom (1993–94), she played the mischievous younger sister of a rising teenage tennis star. Johnson appeared in the 1994 sitcom All-American Girl, which also lasted for only one season. She played DJ's overbearing girlfriend Lisa in the 1995 Roseanne episode "The Blaming of the Shrew", and played Alex Marshall, the daughter of Mel Gibson's character, in the 2000 comedy What Women Want. In 2008, she became a regular on the drama Dirt as Sharlee Cates. In 2009, Johnson appeared in "Omega", the season one finale of Joss Whedon's Dollhouse.