Moynahan grew up in Longmeadow, Massachusetts, along with her two brothers, Andrew and Sean. She is the middle child. 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. Moynahan is also a third cousin of Rosie O'Donnell. 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. She also had a part-time job at a buffet-style restaurant in high school.

She studied acting at the Caymichael Patten Studio in New York City. In 1999 she had her first big role (in a guest appearance) as Natasha in the TV series Sex and the City. Her breakthrough role in a feature film was as Rachel in Coyote Ugly a year later. Since then she has appeared on a number of blockbuster movies, such as The Sum of All Fears, The Recruit, Lord of War, and I, Robot. She most recently starred in the now canceled television series Six Degrees.

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 modeling career. In May 2006, Maxim magazine named her #96 on their annual Hot 100 List.

Moynahan dated NFL quarterback Tom Brady from 2004 until late 2006. On February 18, 2007, Moynahan confirmed to People magazine that she was more than three months pregnant and that Brady was the father. She gave birth to a baby boy, John Edward Thomas Moynahan, at Saint John's Health Center in Santa Monica on August 22, 2007. The baby has his father's first and middle names as his own middle names, though in reverse order. Tom Brady flew to the West Coast, and was present for the birth

--Wikipedia