She is best known for her roles as Sandy Jameson in the television series 7th Heaven, Amy Sanders in Lizzie McGuire and Summer Wheatly in Napoleon Dynamite.

[3] Duff's early career started by making guest appearances on made-for-television films such as True Women and on TV series such as The Amanda Show.

In addition to guest-starring roles on Chicago Hope, Boston Public, and Third Watch, Duff became a familiar face starting in late 2002 as Amy Sanders on Lizzie McGuire.

After making guest appearances on television, Duff received her first role in a feature film when she was cast as Summer Wheatly in Napoleon Dynamite.

In 2005, Duff joined the cast of the television series 7th Heaven, playing Sandy Jameson, best friend to Simon's girlfriend Rose.

In June 2006, Duff joined the Broadway cast of Hairspray, portraying mean girl Amber Von Tussle,[4] and left the role in early October 2006.

She was featured on rapper Kool G Rap's album Half a Klip, providing the background vocals on the track "On the Rise Again" produced by DJ Premier.

She is also featured on song "Babysitting Is a Bum Deal" on the album Family Guy: Live in Las Vegas, singing a duet with Seth MacFarlane as his character Stewie.

[7] When Paris Hilton revealed she had also recorded a version for her debut album, Duff claimed that she had procured the rights from the songwriters prior and attempted to block her from releasing the song.