Peet's first screen performance was a television commercial for Skittles. Her early roles included a guest starring 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 TV 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 met. Her character is notable for seemingly dating two men at once: Jerry and her apparent roommate ("dude"), Lyle. Peet's first role in a widely-released feature film came in 2000, with The Whole Nine Yards, elevating 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.
In 2005, she starred in the play This Is How It Goes, alongside Jeffrey Wright and Ben Stiller. In the same year she also starred in the film A Lot Like Love, with Ashton Kutcher. In February 2006, she was performing in Neil Simon's Broadway production of Barefoot in the Park.
On February 3, 2006, it was announced Peet would be joining the cast of Aaron Sorkin's show Studio 60 on the Sunset Strip, which premiered on NBC on September 18, 2006. She stars alongside Matthew Perry, with whom she worked in The Whole Nine and Ten Yards, and Sarah Paulson, with whom she co-starred in Jack & Jill. In the show, Peet's character Jordan McDeere is the newly appointed president of the NBS.

Peet became engaged to actor/writer David Benioff on July 19, 2005, and later acknowledged her mother was helping her plan a Jewish wedding. On September 15, 2006, Peet's publicist confirmed the two are expecting a child, a daughter sometime in 2007. On the weekend of October 1, 2006, Peet and Benioff married in New York City.