Duff's early career started by making guest appearances on made for television films such as Hope and 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 Saunders/Sanders on the children's television series, Lizzie McGuire, which starred her sister Hilary.
In 2004, Duff made a guest appearances on That's So Raven as Katina Jones. After making guest appearances on television, Duff received her first role in a feature film when she was cast as Summer Wheatley in Napoleon Dynamite. The film earned her first Teen Choice Award win. She then continued acting by making guest appearances, which include Joan of Arcadia and American Dreams. She also lent her voice talent to the Christmas animation film In Search of Santa which also featured sister Hilary. In 2005, Duff joined the cast of the television series 7th Heaven, playing Sandy Jameson, best friend to Simon's girlfriend Rose, who ended up getting pregnant by Martin Brewer (Tyler Hoechlin). 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, where she is credited as co-producer and her mother and sister are credited as producers.
Following Material Girls, Duff appeared in various made-for-television or straight-to-DVD films including Nightmare, My Sexiest Year, Legacy, Backwoods (with Ryan Merriman), Love Takes Wing, Love Finds a Home and My Nanny's Secret. Throughout 2009 and 2010 she appeared in Fear Island, Tug and Slightly Single in L.A.. She lent her voice in the animated film Foodfight! alongside Charlie Sheen, Hilary Duff and Eva Longoria but due to distribution issues the film is yet to have an official release. She also completed work on the independent film Video Girl in which she will be starring alongside Meagan Good and Ruby Dee.
Duff hosted the reality show Legally Blonde The Musical: The Search for Elle Woods, which searched 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. As of 2011, Duff has completed work on A Holiday Engagement in which she will be starring alongside Bonnie Somerville and Jordan Bridges. She has also completed work on Pennhurst, Home Invasion, and recently according to her Twitter she has recently finished filming a pilot for a thriller television show entitled Blackout.
Throughout 2012 she has finished work on films Desecrated, Golden Winter, The Wedding Pact, & All About Christmas Eve. She also voiced Summer Wheatly in the animated version of Napoleon Dynamite, appeared on 2 episodes of the Youtube series Massholes and had a guest role on The Secret Life of the American Teenager. As of 2013, Haylie has completed work on Taken By Grace, Muffin Top: A love Story, and 2 Hallmark films Cupid's Bed and Breakfast & Christmas Belle. Duff's film The Costume Shop has been completed with filming in May 2013 according to the film's twitter.
Duff has recorded numerous singles for various soundtracks with her sister, with most of them appearing on the Disneymania discs. Notable soundtracks are: In Search of Santa, The Lizzie McGuire Movie, A Cinderella Story, and Material Girls. 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. Duff also sang "A Whatever Life" for the Stuck In The Suburbs soundtrack and "Sweetest Pain" for the Raising Helen soundtrack. 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). In addition to singing, Duff has also written and co-written several songs for her sister Hilary's studio albums, Metamorphosis and Hilary Duff, as well as the song "Gypsy Woman", on Hilary's fourth studio album, Dignity. Duff also co-wrote the song "Holiday" on her sister Hilary's Best of Hilary Duff album with Hilary and Ryan Tedder.
In March 2013, Duff has now announced that she has signed a contract with RCA Records and will be working on her debut album. Duff started recording in April 2013.
Duff started her own blog called "Real Girl's Kitchen". Duff dated her boyfriend Nick Zano from October 2007 till November 2012. In November 2012 Duff and Zano were engaged. The duo got married on September 28, 2013.