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. 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, 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