Moving to Los Angeles to pursue acting full-time later that year, Duff promptly landed a role in the direct-to-video sequel “Addams Family Values” directed by Dave Payne. The sub-par film was savaged by the few critics who bothered to see it. Nevertheless, it served as Duff’s entry into Hollywood proper. Fortunately, Duff’s next two projects were better received by reviewers (albeit, equally ignored by audiences). In the critically lauded “Dreams in the Attic” (2000), Duff played Jessica, the lonely niece of an eccentric costume shop proprietor (portrayed by Shelley Duvall) who discovers a chest full of magical dreams and fantasies hidden away in a dusty old attic. Duff next popped up in the comedy short “The Newman Shower” (2001), a coming-of-age teen escapade set in the late 1980’s at a summer camp.

In between these projects, Duff continued to remain busy with guest appearances on television. In addition to guest-starring roles on and “Chicago Hope” (CBS, 1994-2000) “Boston Public” (Fox, 2000-2004), and “Third Watch” (NBC, 1999-2005), Duff became a familiar face starting in late 2002 as Cousin Amy on the kiddie hit, “Lizzie McGuire” (Disney Channel, 2001-2004). The star of that show was, of course, Duff’s own younger sister, Hilary, whose own acting career was just starting to take off at the time. Hilary’s career would quickly eclipse her sibling's in short time, but Haylie would continue to maintain a presence on the small screen.

In 2004, Haylie Duff’s memorable turn as bad girl Katina Jones in an episode of “That’s So Raven”, caught the eye of writer-director Jared Hess, who was seeking an actress to play a similar role in his quirky new independent movie, “Napoleon Dynamite” (2004). The film, which opened to rave reviews, became an instant cult hit and catapulted many of its stars on to bigger and better things. While the success of “Napoleon Dynamite” proved far more of a career boon for its lead, newcomer Jon Heder, critics also took notice of its supporting players, including Duff, Efrem Ramirez, and Tina Majorino. Though hers was the smallest role of the four, Duff’s turn as snob princess Summer Wheatly was nevertheless applauded for giving “Napoleon Dynamite” its true-to-life social satire edge. In 2005, Duff found herself with an incredibly full plate. In addition to appearing with sister Hilary in a multi-episode arc of the CBS drama “Joan of Arcadia”, Duff joined the cast of the long running WB series “7th Heaven” for its tenth and final season. In it, Duff played the role of Sandy Jameson, a pregnant college student who throws young Martin Brewer’s life into turmoil.