Colonel McCarthy was commissioned in the United States Marine Corps in 1998. He completed active duty assignments with 2d Battalion, 8th Marines, deploying with 24th Marine Expeditionary Unit (Special Operations Capable), and the Chemical Biological Incident Response Force, deploying with Combined Joint Task Force-Consequence Management in support of Operation IRAQI FREEDOM 1. He left active duty in 2004 to study law, accepting a commission in the Marine Corps Reserve.
Colonel McCarthy commanded at the company and battalion level with 3d Battalion, 25th Marines, as well as commanding the Marine Detachment of the Joint Enabling Capabilities Command. His staff assignments include service as a Congressional Fellow to the Chairman of the House Intelligence Committee; Future Operations Officer, 22d Marine Expeditionary Unit; Operations Officer, 4th Air Naval Gunfire Liaison Company; and as Executive Officer for 4th Light Armored Reconnaissance Battalion; and later, 25th Marine Regiment. He assumed his current duties as Assistant Division Commander, 4th Marine Division in May of 2024.
Colonel McCarthy is a graduate of the Amphibious Warfare School (DEP), the Marine Corps Command and Staff College (DEP), and the Eisenhower School for National Security and Resource Strategy. He has a Bachelor of Arts degree from the University of Dayton, a Certificate in Legislative Studies from Georgetown University, a Master of Science degree from the National Defense University, and a Juris Doctorate from the Case Western Reserve University. In civilian life, Colonel McCarthy is an attorney and consultant in private practice.