Hewitt was born in Waco, Texas, and is the daughter of Patricia Mae (née Shipp), a speech-language pathologist, and Herbert Daniel Hewitt, a medical technician. Hewitt grew up in Nolanville, Texas. After the divorce of her parents, Hewitt and her older brother, Todd Hewitt, were brought up by her mother.
As a young girl, Hewitt was attracted to music, which led to her first encounters with the entertainment industry. At the age of three, she sang "The Greatest Love of All" at a livestock show. Just a year after that, at a restaurant-dance hall, she entertained an audience with her version of "Help Me Make It Through the Night." By the time she was five, Hewitt already had tap dancing and ballet in her portfolio. At nine, she became a member of the Texas Show Team (which also toured in the Soviet Union). At the age of ten, at the suggestion of talent scouts and winning the title of Texas Our Little Miss Talent Winner, she moved to Los Angeles, with her mother to pursue a career in both acting and singing. In Los Angeles, Hewitt attended Lincoln High School where her classmates included Jonathan Neville, who became a talent scout and recommended Hewitt for her role in Party of Five.