She appeared in several episodes of Lizzie McGuire, That's So Raven and on other TV programs, which include Joan of Arcadia, American Dreams, Boston Public, and Chicago Hope. She played Summer, a stereotypical popular teenager, in the 2004 independent film Napoleon Dynamite. In 2005, Duff joined the cast of the TV show 7th Heaven, playing Sandy Jameson, best friend to Simon's girlfriend and a pregnant woman. Duff was also in a straight to DVD release, Cindy and The Halo Boy, co-starring Frankie Muniz of Malcolm in the Middle fame. In the film, Duff played the leader of a clique of girls in high school who was pregnant with a baby fathered by Muniz's character, nicknamed "Geekling", the leader of a clique of nerdy boys who had wispy mustaches and played the video game "Halo". In summer 2006, Duff joined the Broadway cast of Hairspray, portraying mean girl Amber Von Tussle, and playing the part through the beginning of October 2006. Duff has also starred in Material Girls with her sister Hilary. Duff is also scheduled to shoot a horror film with Crispin Glover, The I Scream Man.

Duff wrote or co-wrote several songs on three of Hilary Duff's albums, Metamorphosis (2003), Hilary Duff (2004), and the song "Gypsy Woman", on Hilary's fourth album, Dignity (2007). She has recorded numerous singles for various soundtracks with her sister, with most of them appearing on the Disneymania discs. She was featured on legendary 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 had supposedly completed work on her debut album Walk the Walk due mid 2008. Duff confirmed that this was false; she is not working on or releasing an album at this time. Duff was involved in a court battle with Paris Hilton over the song Screwed. Both Duff and Hilton had recorded it in 2004, Duff having planned it for her album, and Hilton for it to be released as her debut single, however after what seemed like Hilton had withdrawn from the fight, she was awarded the song by the court, and Duff was left without it. Even though she won the battle for "Screwed", Hilton released "Stars Are Blind" instead, releasing "Screwed" in Asia only.

Duff is currently hosting the reality show Legally Blonde The Musical: The Search for Elle Woods, which is searching for the next actress to take over the lead role in Legally Blonde: The Musical where she was previously in the chorus. She is also an executive producer of the show.

• Slightly Single in L.A. (2009) (pre-production) .... Jill
• A Nanny's Secret (2009) (TV) (filming) .... Claudia
• Love Finds a Home (2009) (TV) (post-production) .... Annie Nelson
• Tug (2009) (post-production) .... Kim
• Love Takes Wing (2009) (TV) (post-production) .... Annie Nelson
• Deep Cove (2008) (post-production) .... Jenna
• Foodfight! (2008) (completed) (voice) .... Sweet Cakes
• Legacy (2008) (V) .... Lana Stephens
• Backwoods (2008) (TV) .... Lee
• My Sexiest Year (2007) .... Debbie
• "7th Heaven" .... Sandy Jameson (32 episodes, 2005-2007)
  ... aka 7th Heaven: Beginnings (USA: rerun title)
  ... aka Seventh Heaven
       - And Away We Go (2007) TV episode .... Sandy Jameson
       - Good News for Almost Everyone (2007) TV episode .... Sandy Jameson
       - Nothing Says Lovin' Like Something from the Oven (2007) TV episode .... Sandy Jameson
       - Some Break-Ups and Some Get-Togethers (2007) TV episode .... Sandy Jameson
       - Inked (2007) TV episode .... Sandy Jameson
        (27 more)
• Nightmare (2007) (TV) .... Molly
• Material Girls (2006) .... Ava Marchetta
• "Joan of Arcadia" .... Stevie Marx (3 episodes, 2005)
       - Shadows and Light (2005) TV episode .... Stevie Marx
       - Independence Day (2005) TV episode .... Stevie Marx
       - Romancing the Joan (2005) TV episode .... Stevie Marx
• "Complete Savages" .... Jessica (1 episode, 2005)
       - Save a Dance for Me (2005) TV episode .... Jessica
• Dishdogz (2005) .... Cassidy

more...