"Served" is a word that does not necessarily apply in an obvious way to the County Militia.
The County Militia were all free, white able-bodied males aged roughly 16-60 (varies by time and place) who lived within a given County. These County residents were obligated to turn out for regular drill and nose-counting (the occasion for making most surviving militia rosters, which also often doubled as tax assessors' lists), which was not considered active-duty service.
The County Militia were organized within each County for defense within the County. Active duty service might include patrolling, guarding prisoners, sometimes garrisoning a local fort or blockhouse, or assisting with escort of supplies bound elsewhere. Most militiamen did not see any active duty. There were many areas where there just was not any military threat by British troops and/or their Tory sympathizers.
Few militia units were sent outside their home Counties for emergency joint action with State Troops and/or the Continental Army, but there were indeed exceptions in strategic areas (such as port cities) and on the Western frontiers.
There are two volumes of the published MD State Archives that have information about MD military during the Revolutionary War. If you go to the MD State Archives website and click to see all volumes, look for Vol. XVIII, which has some useful introductory material. I am sorry I don't recall what the other volume number is, but if you scroll down the list you will find it.