Moynahan grew up in Longmeadow, Massachusetts, along with her two brothers, Andrew and Seán. Her father is a scientist who is currently an administrator at the University of Massachusetts at Amherst. Her mother is a former school teacher and can be seen with her daughter in a 2005 Comcast cable company television advertisement for the Boston area.
She studied acting in New York City. She recently appeared in such feature films as The Sum of All Fears, The Recruit, Lord of War, and I, Robot. She also made guest appearances in the television series Sex and the City.
Known as "Kat" to her friends, Moynahan attended Longmeadow High School and graduated in 1989. In high school, she was active in school athletics, participating in field hockey, basketball and lacrosse.
As a model she was on many magazine covers, including Elle, Vogue, Self, Fitness, New Woman and Glamour. However, she didn't begin to pursue acting until after starting her modelling career.
In May 2006, Maxim magazine named her #96 on their annual Hot 100 List
Personal life
Moynahan dated New England Patriots quarterback Tom Brady from 2004 until Thanksgiving 2006. The couple, who split in December 2006, are expecting a child in late summer of 2007.
