The area now known as Montgomery County was originally a part of the territory north of Charles County. In 1696 parts of Charles County and Calvert County, Maryland were split off and with a large portion of frontier territory formed the new Prince George's County. In turn, in 1748, a portion of Prince George's County produced Frederick County. Montgomery County was formed in 1776 by the splitting of Frederick County. The former Frederick County was subdivided into three; the central portion remained Frederick County, while the western was named Washington County in honor of General (later President) George Washington, and the eastern part was named Montgomery County in honor of another Revolutionary War general, Richard Montgomery.
In 1791, portions of Montgomery County, including Georgetown, were ceded to form the new District of Columbia, along with portions of Prince George's County, Maryland, as well as parts of Virginia that were later returned to Virginia.
Courtesy of http://en.wikipedia.org/wiki/Montgomery_County,_Maryland