Assistant Director McCarthy joined the Naval Criminal Investigative Service (NCIS) in 1981. She has been assigned to a variety of NCIS field positions in San Diego, California; Naples, Italy; Washington, District of Columbia; London, England; Great Lakes, Illinois and Newport, Rhode Island.

In 1987, Special Agent McCarthy became Special Assistant to the Regional Director European Region. She subsequently held the following supervisory positions: Assistant to the Regional Director for Counterintelligence in the European Region, Assistant Special Agent in Charge NCIS Resident Agency London, Assistant Special Agent in Charge Midwest Field Office and Special Agent in Charge Northeast Field Office.

Special Agent McCarthy’s additional assignments included Deputy Assistant Director for Government Liaison and Public Affairs, serving as advisor to the Assistant Director concerning all of NCIS investigations, both criminal and counterintelligence which were likely to elicit congressional interest. During this assignment, she was selected for the Office of Personnel Management "Legis Fellow" program. Assigned to a US Senator’s staff as a Legislative Assistant responsible for law enforcement issues. Also assigned as the NCIS Representative at the Central Intelligence Agency, Counter-Terrorism Center/State Terrorism Branch. Other supervisory positions include Special Agent in Charge Northeast Field Office and Assistant Special Agent in Charge Midwest Field Office.