In 2000, Mastromonaco moved to Boston and got her first job in politics as a staff assistant to Senator John Kerry. For a short time, after 9/11, she worked at a Republican lobbyist group, Richard Berman's American Beverage Institute, as director of membership. In 2002, Mastromonaco was hired as Press Secretary for Congressman Rick Boucher of Virginia, but in December 2002 went back to work for Kerry. In 2004, Mastromonaco was hired as the Director of Scheduling for Senator Kerry's presidential campaign.