She began working for the Manhattan District Attorney's office in 1983, when she was hired by former Manhattan DA Robert Morgenthau. She became a specialist in sex crimes following her first rape case in 1986. In 1991, she became deputy of the Manhattan DA Sex Crimes Prosecution Unit under Linda Farstein, and in 2002, she became the unit's chief following Farstein's retirement. She served as chief of the sex crimes unit for almost ten years before resigning in 2011. Friel appeared in court in the case against former IMF director Dominique Strauss-Kahn, although she was not a member of investigating team.