In 2000, she made her TV debut in two episodes of "Young Americans" (2000), then appeared in "Law & Order: Special Victims Unit" (1999). She made her big screen debut with a small role of Henrietta in Perfume (2001). Monaghan shot to fame in 2002 when she co-starred as Kimberly Woods for one season on the TV series "Boston Public" (2000). After appearances in several supporting roles, she starred opposite Robert Downey Jr. and Val Kilmer in the black comedy Kiss Kiss Bang Bang (2005). Later in 2005, Michelle Monaghan was filming in China, Italy, and the United States on Mission: Impossible III (2006), as the female lead opposite Tom Cruise.
In August of 2005, in Sydney, Australia, Michelle Monaghan married her long-time sweetheart, Peter White, a New York based graphic designer, whom she met at a Manhattan party five years earlier.