Jennifer Love Hewitt was born in Waco, Texas on February 21, 1979. Her mother, Patricia Mae, is a speech-language pathologist, and her father, Herbert Daniel Hewitt, a medical technician. Hewitt grew up in Nolanville, Texas. After the divorce of her parents, Hewitt and her brother, Todd Hewitt, were brought up by her mother. Her first name was given by her brother, while her middle name, "Love", by her mother.

As a young girl, Hewitt was always attracted to music. At the age of three, she sang "The Greatest Love of All" at a small show. 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 Hewitt was five years old, she already had tap dancing and ballet in her portfolio. At nine, she became a member of the Texas Show Team, which was a dance team that toured Russia and Denmark. At the age of ten, she was an international spokesperson for LA Gear shoes. By then she has decided to move to Los Angeles, California, with her mother to pursue a career in both acting and singing.

After moving to Los Angeles, Hewitt appeared in more than twenty television commercials. Her first break came as a child actor on the Disney Channel variety show Kids Incorporated (1989–1991). In 1993, she played Pierce Brosnan's daughter in Running Wilde. Then she acted in several television serials, such as Shaky Ground (1992–1993), Byrds of Paradise (1994), and McKenna (1994–1995).

Hewitt made her film debut through Munchie (1992). However, she became a star after the movie I Know What You Did Last Summer (1997). The film was a great box-office success collecting 125,000,000 U.S. dollars worldwide. She also appeared in the sequel I Still Know What You Did Last Summer (1998), which was not as successful as the first film. Other notable movies include Can't Hardly Wait (1998), The Audrey Hepburn Story (2000), and Heartbreakers (2001).

In 2000, she won the "most popular actress on television" due to her Q-rating (a measurement of a celebrity's popularity) and Nokia chose her to become its spokesperson.

In 2001, she appeared in the music video for the Enrique Iglesias’s song, "Hero." Hewitt wrote the song “ I'm Gonna Love You” for the movie The Hunchback of Notre Dame II and the song won an award for Best Song on DVD Awards. Since September 2005, Hewitt has starred in the television series Ghost Whisperer which became popular in Australia and in the United States.

Hewitt is an honorary godparent of the Audrey Hepburn Children's Fund. She has dated singer-guitarist John Mayer, talk show host Carson Daly, actor/model Kip Pardue, writer Chris Benson, singer-songwriter Rich Cronin and actor Will Friedle. Hewitt began dating Scottish actor Ross McCall in January 2006 and in November 2007, while on vacation in Hawaii, they became engaged.