[1][3] She is the daughter of Irish Americans Mary Bridget (née Moriarty), a former school teacher, and Edward Bradley Moynahan, a scientist and former administrator at the University of Massachusetts Amherst,[4] and has described her family background as Irish-Catholic.

She studied acting at the Caymichael Patten Studio in New York[10] and made her TV debut in 1999 as Natasha in HBO's romantic comedy Sex and the City.

[10] Moynahan made her feature-film debut in the 2000 comedy-drama Coyote Ugly as Rachel, a bartender and dancer in a wild New York bar,[15] a role considered her breakthrough.

[19] Moynahan worked opposite Ben Affleck and Morgan Freeman in the action film The Sum of All Fears, based on Tom Clancy's book of the same name.

[21] Dave Larsen of the Dayton Daily News reported the subplot involving Moynahan and Affleck was "the film's weakest point.

[25] The movie was poorly received,[26] with Mike Clark of USA Today calling it "a less-than-middling melodrama whose subject matter and talent never click as much as its credits portend.

"[27] In 2004, Moynahan worked alongside Will Smith in Alex Proyas' science-fiction movie I, Robot, loosely based on Isaac Asimov's short-story collection of the same name.

Daniel Neman of Richmond Times-Dispatch disliked the film, concluding that Moynahan "turns in an able performance as Dr. Calvin, the convenient character.

[32] Her next movie was 2005's Lord of War, a political crime thriller, where she played Ava Fontaine Orlov, the wife of Nicolas Cage's character.

[34] In September 2006, away from film, Moynahan worked as Whitney Crane in the ABC television drama series Six Degrees co-starring with Jay Hernandez, Erika Christensen, Hope Davis, Dorian Missick, and Campbell Scott.

[44] In December 2008, Moynahan guest-starred in two episodes of the ABC television comedy-drama Eli Stone as the titular character's (Jonny Lee Miller) ex-girlfriend.

The next year, Moynahan worked with Aaron Eckhart, Michelle Rodriguez and Michael Peña in the action science-fiction feature Battle: Los Angeles (2011).

[50] In November 2009, Moynahan signed a deal with Garnier to appear in television and print advertising promoting their Ultra-Lift skincare products.