Born in Brooklyn, McCarthy worked as a Licensed Practical Nurse and lived with her family in the Long Island town of Mineola, a suburban area near New York City, with her ex-husband, Dennis, and son, Kevin. On December 7, 1993, her ex-husband was killed and her son was injured on a Long Island Rail Road commuter train returning to Mineola, when a gunman, Colin Ferguson, opened fire on random passengers. McCarthy responded to the tragedy by launching a campaign against gun violence that eventually propelled her to Congress. She defeated Republican incumbent Dan Frisa, who had repeatedly voted against gun-control legislation, by 15 points. The Fourth District had been in Republican hands since 1953, but has supported Democrats for president since 1992. She faced a close fight for reelection against state assemblyman Gregory Becker, but didn't face another serious opponent until 2004. That year, she faced Hempstead mayor James Garner, who served as President of the U.S. Conference of Mayors in 2003. It was originally believed to be a tight race, but McCarthy defeated Garner 63 o 37åP> McCarthy serves on the Education and Workforce Committee, and the Financial Services Committee. She is also one of the most vocal supporters in the House for reinstating a Federal ban on semi-automatic firearms commonly referred to as assault weapons. In addition to her efforts to reduce gun violence, McCarthy is especially active in issues relating to education and public health.

Even though McCarthy has always served in the House as a Democrat, she was still registered as a Republican until 2003.