
As a singer, Hewitt has been signed by Atlantic Records and Jive Records. She is primarily known for her recordings in the pop genre and has a contralto vocal range. To date, her most successful single on the Billboard Hot 100 is the 1999 release "How Do I Deal", which peaked at #59. In addition, she has contributed music to the promotion or soundtracks of acting projects.

In addition to acting, Hewitt has also served as a producer on some of her film and television projects. She can be seen on the CBS television program Ghost Whisperer as Melinda Gordon, a young woman who can communicate with ghosts. She won a Saturn Award in 2007 and 2008 for Best Actress on Television

Hewitt was born in Waco, Texas, 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, California, 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 later became a talent scout and recommended Hewitt for her role in Party of Five.

Hewitt is an honorary godparent of the Audrey Hepburn Children's Fund.
She has dated singer-guitarist John Mayer and professional kayaker Brad Ludden among others.
Hewitt was engaged to Scottish actor Ross McCall, whom she met when he made an appearance on Ghost Whisperer. After a year of dating, they became engaged In November 2007, while vacationing in Hawaii. However, on January 5, 2009, People magazine reported that Hewitt and McCall called off their engagement. She later began a relationship with Ghost Whisperer co-star Jamie Kennedy; the two were reported to be engaged July 21, 2009, but the couples' representatives and Kennedy himself later stated that this had not happened.
Hewitt told Craig Ferguson she was a soccer fan, and she is a supporter of Celtic F.C on The Late Late Show with Craig Ferguson.
Paparazzi photographs secretly taken of Hewitt on vacation in a bikini led to harsh criticism of her body by bloggers. Hewitt responded: "I've sat by in silence for a long time now about the way women's bodies are constantly scrutinized.... What I should be doing is celebrating some of the best days of my life and my engagement to the man of my dreams, instead of having to deal with photographers taking invasive pictures from bad angles." She posed on the cover of Us Weekly 10 weeks later, having lost 18lb since the photos were taken. She said she had lost the weight to boost her energy levels, while her trainer claims that she said the weight loss had nothing to do with critical comments. Hewitt later defended Jessica Simpson for receiving similar criticism saying, "It gets silly. We're all supposed to look how we're supposed to look. We're made to be different people."