​The Maryland National Guard traces its lineage to colonial militias that were raised​ by the 1632 Maryland Charter. The approximately 6,000 National Guard members typically hold civilian jobs or attend college while maintaining military training part-time. Guard members are found all across Maryland.

The Maryland militia gained an illustrious reputation during the Revolutionary War, notably at the 1776 Battle of Long Island, where Maryland militia carried out a desperate rearguard action to protect the retreating colonial army. Many historians believe this to be the origin of "The Maryland Line" nickname for the Maryland Army National Guard, as well as the "Old Line State" nickname. The Maryland militia or National Guard has taken part in almost every major U.S. armed conflict.

The Maryland National Guard consists of the Maryland Army National Guard (MDARNG) and the Maryland Air National Guard (MDANG), whose purposes are to respond to state disasters and needs, and to support assigned missions when federally activated. Although federally funded, the Maryland National Guard is subordinated to the state except when called into federal service as specified by law, unlike active duty or reserve units. This distinction allows the Governor to call up the Maryland National Guard to respond to disasters and crises. In the 21st century, Maryland National Guard personnel have been called to assist in a diverse array of situations, ranging from search and rescue after storms to removing debris after Hurricane Sandy to transporting patients during the COVID-19 pandemic.

The MDARNG and MDANG fall under the Maryland Military Department, which is headed by The Adjutant General (TAG), who is appointed by the Governor.

The Military Department is responsible for building and maintaining the armories, readiness centers, and other facilities used by the Maryland National Guard and is made up of the following agencies:

• State Operations - The State Operations section manages many of the fiscal and administrative duties of the Military Department. The Freestate ChalleNGe program and Honor Guard also fall under this group.
• Maryland Air National Guard - The MDANG provides air combat forces and airlift aircraft, as well as emergency support, to the U.S. Air Force and other unified commands.
• Maryland Army National Guard - The MDARNG provides combat troops to the U.S. Army; engineer, transportation, and medical units; and military police for state service.
• Maryland Defense Force (MDDF) - The Maryland Defense Force is separate and distinct from the Maryland National Guard and is composed of officers -commissioned or assigned- and all able-bodied citizens of the state who volunteer and are accepted for service. They may be ordered to active duty in the state by the Governor or whenever the National Guard has been called into the military service of the United States; however, the MDDF is entirely state funded and cannot be federalized or ordered to be deployed outside of Maryland.

The Maryland Military Department owns and maintains 38 Readiness Centers, Military Reservations, and other properties in 14 counties and Baltimore City. Readiness Centers allow local National Guard personnel to assemble, train, and maintain equipment. Warfield Air National Guard Base, which shares a runway with Martin State Airport, hosts the only flying mission of the MDANG. Organized by county and municipality, these properties are:

• Allegany County: Cumberland - Capt. Thomas Price Readiness Center
• Anne Arundel County: Annapolis - Lt. Col. (MD) E. Leslie Medford Readiness Center; Glen Burnie - First Regiment Armory
• Baltimore City: Fifth Regiment Armory; Lt. Col. Melvin H. Cade Readiness Center
• Baltimore County: Catonsville - Maj. Gen. William J. Witte Readiness Center; Dundalk - CSM Jerome M. Grollman Readiness Center; Glen Arm - Gunpowder Military Reservation; Glen Arm - Maj. Gen. William C. Purnell Readiness Center; Martin State Airport - Warfield Air National Guard Base; Parkville - CW4 Melvin Sherr Readiness Center; Pikesville Military Reservation; Pikesville - Maj. Gen. (MD) John Purley Cooper, Jr. Armory; Reisterstown - Camp Fretterd Military Reservation; Reisterstown - Camp Fretterd Readiness Center; Towson Readiness Center; Towson - Maj. Gen. (MD) Harry C. Ruhl Readiness Center
• Carroll County: Westminster - Maj. Gen. Henry C. Evans Readiness Center
• Cecil County: Elkton - Lt. Col. James Victor McCool Readiness Center
• Charles County: La Plata - Brig. Gen. William Smallwood Readiness Center
• Frederick County: Frederick - Capt. Michael Cresap Readiness Center
• Harford County: Aberdeen - Army Aviation Support Facilities - Col. William C. Baxter Hangar & Edward J. Weide Airfield, Aberdeen Proving Ground; Aberdeen - Edgewood Armory; Bel Air - Lt. Gen. (MD) Milton A. Reckord Armory; Edgewood - Lauderick Creek Training Site; Havre de Grace Military Reservation
• Howard County: Ellicott City - Brig. Gen. Thomas B. Baker Readiness Center
• Montgomery County: Olney - Col. Henry A. Cole Readiness Center; White Oak - Maj. Gen. George M. Gelston Readiness Center
• Prince George's County: Cheltenham - Congressman Steny H. Hoyer Readiness Center; Greenbelt - Maj. Gen. (Brevet) John R. Kenly Readiness Center; Laurel - Pvt. Henry Costin Readiness Center
• Queen Anne's County: Queen Anne - Col. Victor P. Gillespie Readiness Center
• Talbot County: Easton - Brig. Gen. Louis G. Smith Readiness Center
• Washington County: Cascade - Maj. Gen. Boyd M. Cook Readiness Center, Fort Ritchie; Hagerstown - Brig. Gen. (MD) Randolph Millholland Readiness Center; Hancock - Brig. Gen. Thomas B. Baker Training Site
• Wicomico County: Salisbury - CSM Blair Lee Crockett Readiness Center