Lt. General Dennis M. McCarthy graduated from the University of Dayton, and was commissioned a Second Lieutenant in 1967.  Upon graduation from The Basic School, he was transferred to the 9th Marine Amphibious Brigade in the Republic of Vietnam where he served as a Platoon Commander and Communications Officer in First Battalion, 13th Marines.

Upon return to the United States in 1969, he served as an assistant S-3 at Quantico, Virginia and on Inspector-Instructor duty with the 25th Marine Regiment in Worcester, Massachusetts.  He was then selected to attend law school on the Excess Leave Program. After graduation, he served as a Judge Advocate at Camp Lejeune, North Carolina. After promotion to Major, he resigned his Regular commission in 1978 and transferred to the Marine Corps Reserve.

As a Reserve officer, he commanded the Third Battalion, 25th Marines and was Executive Officer of the 25th Marine Regiment. He commanded Headquarters Detachment 2, 4th Marine Division and was called to active duty during Operation Desert Shield/Desert Storm for duty at Twentynine Palms, California as Director of the Tactical Exercise Evaluation Control Group. He then served as G-3, 4th Marine Division in 1991, and commanded U.S. Joint Task Force, Chile in 1992. He was selected for promotion to the grade of Brigadier General in 1992.

From 1993 until 1995, General McCarthy was Commanding General of the Marine Corps Reserve Support Command. In 1995 he assumed command of the Augmentation Command Element of the First Marine Expeditionary Force. He was concurrently assigned as Deputy Commander, I MEF. He was promoted to Major General on October 1, 1996, and was appointed to the Defense Reserve Forces Policy Board.In 1997, General McCarthy assumed command of the Third Marine Division, becoming the first Reserve General Officer to command an active duty Marine Division. He served in that post from July through October. He was then assigned to the United States Atlantic Command, where he served as the Vice Director of Operations, and then in 1999 as Director of Operations & Plans. He served as the Director, Reserve Affairs Division, Headquarters, U.S. Marine Corps, from 1 Jan 2000 until May 14, 2001. On June 1, 2001, he was promoted to the grade of Lieutenant General. He became Commander, Marine Forces Reserve, on June 2, 2001 and as Commander, Marine Forces North, on September 13, 2004.  General McCarthy resigned from the U.S. military in 2005.