In 2003, Hilary Duff received her 1st major role in a feature film when Hilary Duff was cast alongside Frankie Muniz in Agent Cody Banks. Agent Cody Banks film received positive reviews and was successful enough to spawn a sequel, in which Duff did not participate. The same year, in 2003, Hilary Duff reprised her role as Lizzie McGuire for The Lizzie McGuire Movie, which exceeded box office expectations, earning $42.6 million at the US box office and $55 million worldwide. It received mixed reviews, with certain critics calling it "an unabashed promotion of Duff’s image, just as Crossroads was for Spears", while other reviews were generally positive and encouraging. Later that year, Hilary Duff played one of the 12 children of Steve Martin and Bonnie Hunt in the family film Cheaper by the Dozen, which remains her highest grossing film to date. Hilary Duff reprised her role in the sequel Cheaper by the Dozen 2 (2005), which failed to be as successful as the original film and was panned by critics.
Hilary Duff's first full-length studio album, Metamorphosis (2003), reached #1 on the U.S. and Canadian charts and had sold over 3.7 million copies by May 2005. The lead single, "So Yesterday" (co-written and produced by The Matrix), was a top ten hit in several countries; its follow-up, the Laguna Beach theme song "Come Clean", became Hilary Duff's first top 40 U.S. hit and reached the top 20 in the UK, Australia, and New Zealand. The video was nominated in the category of MTV Video Music Award for Best Pop Video at the 2004 MTV Video Music Awards. The 3rd single, "Little Voice", was not released in the U.S. and was a minor hit in Canada and Australia. In late 2003, Hilary Duff embarked on her first concert tour, the Metamorphosis Tour, and later the Most Wanted Tour. Most shows scheduled in the major cities were completely sold out.
Hilary Duff also made several guest appearances in television shows, Hilary Duff's 1st as a sick child in the medical drama Chicago Hope in March 2000. In a 2003 episode of George Lopez, Hilary Duff had a role as a makeup salesperson; Hilary Duff later reappeared in the show in 2005 as Kenzie, a feminist poet friend of the character Carmen (Masiela Lusha). In 2003, Hilary Duff acted opposite her sister Haylie in American Dreams, while in 2005, Hilary Duff played a classmate and idolizer of the title character of Joan of Arcadia.