Her first significant role was playing a young and impressionable model in the Australian 1994 film Sirens. Soon afterwards she moved to Los Angeles and had guest roles on several TV shows, and a permanent role in Nick Freno: Licensed Teacher, before landing a role in the film Scream 2. During this time in the United States, de Rossi worked diligently to replace her native Australian accent with her current General American one.
She attracted international attention when she joined the main cast of the Ally McBeal TV series in 1998, playing lawyer Nelle Porter. She remained with the show until its end in 2002. In 2001, she starred in Who Is Cletis Tout? with Christian Slater.
From 2003–2006, de Rossi starred as Lindsay Bluth Fünke on Fox Television's critically acclaimed, Emmy-winning series Arrested Development.
She also portrayed John F. Kennedy, Jr.'s wife, Carolyn Bessette-Kennedy, in the made for TV movie America's Prince: The John F. Kennedy Jr. Story in 2003. In 2005 she portrayed a fortune-teller named Zela in the Wes Craven thriller Cursed. From 2007–2008, de Rossi appeared in Nip/Tuck's fifth season as Julia McNamara's girlfriend Olivia Lord.
In 2009 and 2010, de Rossi played the high-strung and controlling Veronica Palmer on the ABC show Better Off Ted. De Rossi will reprise her role as Lindsay Bluth Fünke in 2012's film adaptation of Arrested Development. In 2011 she appeared in Better Off Ted cast member Malcolm Barrett's music video for "Revenge of the Nerds", spoofing her character Veronica in a cameo (alongside other cast members of the cancelled series).
She ranked 69th in Stuff's 100 Sexiest Women, 31 in Femme Fatales' Sexiest Women of 2003 list, 24 in Maxim's 100 Sexiest Women List in 2004, and in late 2006, the magazine Blender listed her as one of the hottest women of film and TV. In May 2007, she was featured as one of 100 Most Beautiful in a People magazine special edition. TV Guide included her and Ellen DeGeneres in their Power A-List couples in 2007.
In February 2012 it was announced that ABC had ordered a pilot for a new drama series The Smart One, executive produced by Ellen DeGeneres which would feature de Rossi in a leading role. The actress will star as a "brilliant and successful woman who begrudgingly goes to work for her less-brainy but more popular sister – a former beauty queen, weather girl and now big-city Mayor."