Peet's first screen performance was a television commercial for Skittles. Her early roles included a guest role on the television series Law & Order. She made her film debut in Animal Room (1995). Peet maintained a steady acting career in relatively obscure indie movies. Her first major role was as "Jack" in the 1999 WB network series Jack & Jill (which aired for two seasons). She also appeared in the eighth-season finale of Seinfeld ("The Summer of George") as a waitress whom Jerry Seinfeld meets. Peet's first role in a widely-released feature film came in 2000, with The Whole Nine Yards, which, together with the cult film Whipped, helped to elevate her status from supporting actress to lead. That same year, she was voted one of the 50 Most Beautiful People in the World by People magazine. Peet was also in the movie Saving Silverman with Jack Black and Steve Zahn. She also starred in Something's Gotta Give in 2003, playing Diane Keaton's daughter, and, at one point, Jack Nicholson's girlfriend.

In 2005, Peet appeared in the play This Is How It Goes, filling in for Marisa Tomei at the last minute after six days of rehearsal. In the same year, she also co-starred in the films Syriana alongside Matt Damon and George Clooney, and A Lot Like Love, with Ashton Kutcher. In February 2006, she was performing in Neil Simon's Broadway production of Barefoot in the Park.

Peet was a member of the cast of the television series Studio 60 on the Sunset Strip, which premiered on NBC on September 18, 2006. She starred with Matthew Perry, with whom she worked in The Whole Nine Yards and The Whole Ten Yards, and Sarah Paulson, with whom she co-starred in Jack & Jill. In Studio 60, Peet's character Jordan McDeere was the newly-appointed president of the National Broadcasting System (NBS). In 2006, she also starred along with Dermot Mulroney in Griffin and Phoenix, where she played a terminally-ill woman living life to the fullest. In 2007's The Ex, a comedy co-starring Zach Braff, Peet played an attorney who stays home to raise a new baby. In 2008, Peet went on to play an FBI agent in The X-Files: I Want to Believe. She is also coming out in the soon to be Hollywood hit, "2012", a sure crowd pleaser.

• 2012 (2009) (post-production) .... Kate
• Untitled Nicole Holofcener Project (2009) (post-production)
  
• What Doesn't Kill You (2008) .... Stacy Reilly
• The X Files: I Want to Believe (2008) .... ASAC Dakota Whitney
  ... aka The X-Files (USA: promotional abbreviation)
  ... aka The X-Files Movie (USA: promotional title)
  ... aka X-Files: Je veux y croire (Canada: French title)
• $5 a Day (2008) .... Maggie
• Martian Child (2007) .... Harlee
• Terra (2007) (voice) .... Maria
  ... aka Battle for Terra (International: English title)
• "Studio 60 on the Sunset Strip" .... Jordan McDeere (22 episodes, 2006-2007)
  ... aka Studio 60 (USA: informal alternative title)
       - What Kind of Day Has It Been (2007) TV episode .... Jordan McDeere
       - K&R: Part 3 (2007) TV episode (credit only) .... Jordan McDeere
       - K&R: Part 2 (2007) TV episode (credit only) .... Jordan McDeere
       - K&R: Part 1 (2007) TV episode .... Jordan McDeere
       - Breaking News (2007) TV episode .... Jordan McDeere
        (17 more)
• Fast Track (2006) .... Sofia Kowalski
  ... aka The Ex (USA: new title)
• Griffin & Phoenix (2006) .... Phoenix
• Syriana (2005) .... Julie Woodman
• A Lot Like Love (2005) .... Emily Friehl
• Melinda and Melinda (2004) .... Susan
• The Whole Ten Yards (2004) .... Jill
• Something's Gotta Give (2003) .... Marin
• Identity (2003) .... Paris
• Whatever We Do (2003) .... Patty