
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), where she was credited as just Love Hewitt. During this time she danced in and sang all the songs for a live action video called Dance! Workout With Barbie released by Buena Vista.
In 1993, she played Pierce Brosnan's daughter in a pilot for NBC called Running Wilde, which featured Brosnan as a reporter for Auto World magazine whose stories cover his own wild auto adventures. However, the series wasn't picked up and the pilot never aired. Hewitt later had roles in several short-lived television series, such as Fox's Shaky Ground (1992–1993), ABC's The Byrds of Paradise (1994), and McKenna (1994–1995), and finally became a young star after landing the role of Sarah Reeves on the popular Fox Television show Party of Five (1995–1999). She assumed the role of Sarah after joining that show during its second season and continued it on the short-lived Party of Five spin-off, Time of Your Life (1999), which she also co-produced. The show was cancelled after only half a season.
Hewitt made her film debut in the independent film Munchie (1992). She became a film star after a lead role in the horror film I Know What You Did Last Summer (1997), which enjoyed great box-office success (125,000,000 U.S. dollars worldwide). The film made Hewitt and her co-stars Freddie Prinze Jr., Ryan Phillippe, and Sarah Michelle Gellar some of the most popular young stars in the USA. She also appeared in the sequel I Still Know What You Did Last Summer (1998), which, though ultimately not as successful as the first film, took in more money on its opening weekend. Other notable film roles have included the high-school comedy Can't Hardly Wait (1998) and a starring role with Sigourney Weaver in the romantic comedy Heartbreakers (2001).

In 2001, she appeared in the music video for the Enrique Iglesias song, "Hero," as the singer's love interest.
Hewitt wrote "I'm Gonna Love You" for the movie The Hunchback of Notre Dame II because, although she was a well-known singer at the time, her character Madellaine was the only character who didn't sing in the movie. The song won Best Song at the DVD Awards.
Since September 2005, Hewitt has starred in the television series Ghost Whisperer. In Australia, Ghost Whisperer has been popular since its introduction; in the United States, the show averages 9–11 million viewers for each new episode. Hewitt has also auditioned for many roles. She was asked to play Juliet in Romeo + Juliet, but the director felt she wasn't modern looking. The role went to Claire Danes. She also had to give up the role of Darlene in Brokedown Palace because of scheduling conflicts; that role went to Kate Beckinsale.
Hewitt and Jamie Kennedy are starring in "Cafe", an indie drama from writer-director Marc Erlbaum. Alexa Vega, Madeline Carroll, Daniel Eric Gold and Hubble Palmer also appear as part of an ensemble of Philadelphia residents who frequent the local West Philly Grounds coffee shop where Hewitt's character Claire works. The film began shooting on May 11, 2009.
Hewitt and Jamie Kennedy are starring in "Cafe", an indie drama from writer-director Marc Erlbaum. Alexa Vega, Madeline Carroll, Daniel Eric Gold and Hubble Palmer also appear as part of an ensemble of Philadelphia residents who frequent the local West Philly Grounds coffee shop where Hewitt's character Claire works. The film began shooting on May 11, 2009.

In 1991, Meldac funded the recording of Hewitt's first album Love Songs, when she was just 12. The album was only released in Japan in 1992 where Hewitt became a pop star. Her explanation for her success in Japan is that the Japanese "love perky music. The poppier the music, the better."
After she joined the cast of Party of Five in 1995, she signed to Atlantic Records, who rushed her first single and second album, Let's Go Bang, out in October.
Juggling her music career with her acting career, she recorded her follow-up in 1996. The first single, "No Ordinary Love," failed to chart and led to the album doing the same. Atlantic dropped Hewitt, who didn't return to the music scene for three years.
In 1999, she recorded the single "How Do I Deal" for the I Still Know What You Did Last Summer soundtrack. The song became Hewitt's first charting single by climbing to #59 on the Hot 100 and #36 on the Top 40 Mainstream. It also reached #8 in Australia.

Since 2003, Hewitt hasn't actively done anything in the music industry, but a compilation called Cool with You: The Platinum Collection was released in Asia.
In addition to starring in the 2004 film If Only, Hewitt also co-wrote and performed two songs for its soundtrack: "Love Will Show You Everything" and "Take My Heart Back."
She also appeared in the 2004 made-for-television musical version of A Christmas Carol, performing the singing role of Ebenezer Scrooge's fiance Emily.
In May 2009, she confirmed she was working on a country album. The album is due for a late 2009 or early 2010 release date, a country label has not been announced yet.