Finding herself in heavy demand for print ads, Monaghan subsequently dropped out of college her junior year and decided to take up modeling full-time. Within a year, Monaghan left Chicago and moved to New York where she posed for a variety of catalogs, magazines, and clothing designers. By 2000, Monaghan appeared well on her way to a having long, lucrative career working the runways. As it happened, however, Monaghan found herself re-evaluating her career plans once more when she developed an interest in acting.

Monaghan eventually landed a few minor parts in television, most notably on the short-lived WB series “Young Americans” in 2000 and NBC’s “Law & Order: Special Victims Unit” in 2001. These roles, while relatively small, did at least manage to get her work on the big screen. Monaghan’s feature debut was in 2001’s “Perfume” directed by Michael Rymer. While Monaghan’s performance escaped the notice of audiences (as did the movie, itself), she was able parlay it into a slightly showier role in director Adrian Lyne’s romantic thriller, “Unfaithful” in 2002. In the meantime, Monaghan also kept busy by appearing in a number of television commercials.

Monaghan’s big break, however, came late in 2002; when she was added to the cast of David E. Kelley’s hit series, “Boston Public” (FOX, 2000-04). Hand-picked by executive producer Kelley for the role of idealistic, young high school teacher Kimberly Woods, Monaghan garnered positive attention from critics and viewers alike during her two year run on the show.

Following “Boston Public”’s cancellation, Monaghan returned to the silver screen in a series of high profile features starting with “It Runs in the Family” (2003), opposite Michael Douglas, “Winter Solstice” (2004) and the blockbuster smash “The Bourne Supremacy” (2004). As her gigs grew in prestige, so too did Monaghan’s parts: she turned up in the Brad Pitt-Angelina Jolie romantic comedy “Mr. And Mrs. Smith” (2005), played one of Charlize Theron's female miner co-workers and fellow victim of sexual harassment in "North Coutnry" (2005) and she landed her first major leading role in a studio film opposite Robert Downey Jr. and Val Kilmer in the action thriller "Kiss Kiss Bang Bang" (2005). Monaghan is scheduled to appear in 2006's “Mission: Impossible 3” (2006) opposite Tom Cruise, a role she picked up after Scarlett Johanssen withdrew from the project.