Peet continued landing television roles, including a two-episode arc on NBC's "Law & Order" (NBC, 1990- ) in 1995 and an appearance the following year in "The Single Guy" (NBC, 1995-97). Prior to her regular starring turn in The WB's twenty-something romantic comedy series "Jack & Jill" (1999-2001), Peet's best-remembered TV role was likely that of Lanette, one of Jerry's especially demanding girlfriends-of-the-week on a 1997 episode of NBC's monumentally popular sitcom "Seinfeld” (NBC, 1989-1998). That same year, she gave a disarming performance as the hippie-like Julia, a young concerned teacher and temporary guardian of abused child "Ellen Foster" (Jena Malone) in the moving CBS TV-movie adaptation of Kaye Gibbons' novel. Before taking on the role of New York City's somewhat unpolished new arrival Jacqueline 'Jack' Barrett on "Jack & Jill,” she disappeared from the small screen for a time to undertake several film projects.

Peet's movie career commenced in 1994 when she filmed a featured role in the independent drama "Grind,” starring Billy Crudup and Adrienne Shelly. The film wouldn't see release until 1997, so film audiences weren't introduced to the actress until her memorable supporting role as Jennifer Aniston's sister in "She's The One" (1996). The following year, Peet took on a featured role in the independent ensemble comedy "1999,” set at a New Year's Eve party, then went on to appear as Donnie Wahlberg's jilted ex-girlfriend in the gritty Boston-set drama "Southie" (1998). She impressed as co-star of the independent "Origin of the Species" (1998), and cut a surprisingly strong and sympathetic figure as an actress and aerobics instructor who is diagnosed with HIV in the formulaic drama "Touch Me" (1998). Peet would next take a small and uncharacteristically malicious role as the bewitched Sean Patrick Flanery's former girlfriend in the 1999 Sarah Michelle Gellar starrer "Simply Irresistible.” Her busy filming schedule from 1998 on led to a spate of late 1999-early 2000 releases featuring the young up and comer. Among them was "Body Shots,” a "Rashomon"-inspired ensemble piece chronicling different takes on a wild night by a group of eight young partiers. While the film was disappointingly executed, Peet's performance as the down-to-earth Jane (again opposite Flanery) made the character especially relatable, and gave audiences a peek at her notable acting capabilities.

Peet continued her upward climb, breaking into higher-profile features, including her turn in the Jacqueline Susann biopic "Isn't She Great?" (2000), starring Bette Midler as the popular novelist and socialite. She also starred opposite Bruce Willis and Matthew Perry as a dental hygienist with mob ties in the comedy "The Whole Nine Yards" (2000)—along complete with a stunning nude scene involving firearms that demonstrated her flair for comic timing. The actress later reprised the role in the unnecessary sequel "The Whole Ten Yards" (2004). Not forsaking smaller independents, Peet acted in the suicide-themed black comedy "Jump,” which premiered at the 1999 Los Angeles Independent Film Festival, then had a starring turn as a Nina, a blonde beauty vying with another captivating Nina (Cara Buono) for the affections of a writer ("Body Shots" co-star Ron Livingston) in the acclaimed 1999 festival-run feature "Two Ninas.” Something of a fixture on the independent film scene, Peet later starred in "Whipped" (2000), playing a woman simultaneously carrying on relationships with three self-absorbed playboys, much to their surprise.

The unfunny mega-dud "Saving Silverman" cast her as Jason Biggs' gorgeous but shrewish fiancé who inspires his buddies (Jack Black and Steve Zahn) to go to absurd lengths to break them up. in 2002 Peet snagged secondary parts in a pair of watchable thrillers, "High Crimes" and "Changing Lanes," before getting one of her best roles to date in writer-director Burr Steers' seriocomic "Igby Goes Down," in which she played Rachel, the icily beautiful but troubled young mistress of the distant father (Jeff Goldblum) of teen Igby (Keiran Culkin). She next appeared in the mysterioso hit thriller "Identity" (2003) as one of the group of seemingly unrelated people stranded at a roadside motel who realize they are being killed off one by one. Peet exhibited a tremendous amount of appeal in an underdeveloped supporting role in the romantic comedy "Something's Gotta Give" (2003), playing the pert young girlfriend of an aging Lothario (Jack Nicholson) who against his will finds himself attracted to the free spirit's tightly wound mother (Diane Keaton).

Peet had an amusing supporting turn as Will Ferrell's career-obsessed filmmaker wife in writer-director Woody Allen's "Melinda and Melinda" (2005), and delivered one of her most beguiling and well-etched performances to date in the romantic comedy "A Lot Like Love" (2005), in which she plays an edgy, romantically challenged aspiring actress who finds herself diverted every few years by his encounters with an earnest young man with ambitious plans for his life (Ashton Kutcher) who, though their timing always seems to be off, just might be her soul mate. She also had a nicely etched supporting turn in the political potboiler "Syriana" (2005) as the devastated young wife of an oil analyst (Matt Damon) whose family loss leaves him obsessed with helping an oil rich nation being fought over by many political corners. That year Peet also earned strong reviews for her turn in New York's Public Theater production of "This Is How It Goes," Neil Labute's play about an interracial love triangle that she joined just six weeks before opening night. Meanwhile, Peet started 2006 with a whimper, starring as the oddly adorable newlywed, Corie Bratter, in Neil Simon’s valentine to first wife Joan Baim, “Barefoot in the Park.” Critics admonished the Broadway production for being dated and unfunny, while slapping Peet’s wrists for trying to hard with her performance.

Returning to the small screen, Peet joined an ensemble cast for Aaron Sorkin’s highly anticipated, but ultimately disappointing behind-the-scenes look at a late night sketch show, “Studio 60 on the Sunset Strip” (NBC, 2006-2007). Peet portrayed incoming network president Jordan McDeere, hired by the brass to save the network, especially the long-running “Saturday Night Live”-like show that has fallen into abject irrelevance. Cool, calm and whip-smart, McDeere’s solution is to rehire two fired producers (Matthew Perry and Bradley Whitford) while she does routine battle the network’s authoritarian corporate boss (Steven Weber). “Studio 60” entered the fall season with high expectations, but the hysteria quickly died when audiences began tuning out in droves. Though the show was picked up for a full season, low ratings gave the impression that “Studio 60” might be off the 2007-2008 schedule. Hedging her bets, Peet returned to features to play a high-powered corporate attorney who quits her job to have a baby, forcing her underachieving, but loving husband (Zach Braff) to pick up the slack in the derivative romantic comedy, “The Ex” (2007).