Johnson was born in Camarillo, California, the daughter of Nancy (née Spruiell), a writer, and Clifford Johnson, a ship's captain. She played the role of Christine Ellen "Chrissy" Seaver on the television show Growing Pains from 1990 to 1992. She also appeared in the 1994 situation comedy All American Girl and was the voice of Gretchen Grundler on Disney's Recess.
She played Mel Gibson's daughter in the 2000 romantic comedy What Women Want. On the Teen Titans animated series, she voices Terra during the show's second and fifth seasons. She also voices Jinmay on Super Robot Monkey Team Hyperforce Go!. She formerly held the title Miss Jr. Michigan.