After graduating from Longmeadow High School, Moynahan pursued a modeling career, though admits she never read fashion magazines growing up. She had accompanied a friend to a modeling audition in Springfield, Massachusetts and was signed by the modeling agency in place of her friend. She began her career appearing in department store catalogs in Springfield, during which time, she attended the University of Massachusetts Amherst. In her freshman year, Moynahan moved to New York at age 18 and, a year later, began appearing in magazines such as Vogue and Elle, as well as appearing on covers of other widely known magazines. In an interview given in July 2004, Moynahan, discussing her early work, said: "It was a crazy world that paid a lot of money. I liked being a model, but I knew it would never last, so I looked into acting." During that time, she began doing soap and shampoo commercials, in addition to taking acting and art classes, which led her to pursue a career in acting. She studied acting at the Caymichael Patten Studio in New York and, in 1999, made her television debut as Natasha in HBO's romantic comedy Sex and the City. She would later have a recurring role in the show, until the divorce of her character from Mr. Big (Chris Noth). The following year, she appeared in smaller film roles, including parts in In the Weeds and Whipped.