McMonagle was commissioned in the United States Marine Corps as a Second Lieutenant after graduation from University of Notre Dame in 1953.

McMonagle served as rifle company commander with 2nd Marine Division.

As a Lieutenant Colonel, McMonagle was assigned command of the 2nd Battalion, 3rd Marines and deployed to Republic of Vietnam in 1968.

In 1980, McMonagle was Deputy Commanding General of Marine Corps Recruit Depot San Diego.

[5] McMonagle relinquished command to Major General John P. Monahan on July 19, 1988 and retired from active duty after 35 years of service.