There are 95 counties now existing in Virginia. The General Assembly has created subordinate county and city jurisdictions for nearly 400 years, and other counties have been dissolved, converted into cities, or lost to the Northwest Territory, Pennsylvania, Kentucky, and West Virginia. Several of the remaining 95 counties once had different names, and every remaining county has had its boundaries altered at one time or another.
Colony-wide government came first, starting with governance at Jamestown by a private joint-stock company. The Virginia Company initiated the Virginia legislature in 1619. The General Assembly established four "incorporations," and starting in 1634 created the counties. Forming new units of government with county courts helped the legislators offload to others some of the responsibilities/workload for handling land ownership changes, processing wills, and dealing with minor crimes. Similarly, creating parishes with vestry for Anglican churches helped offload the burden of providing social services for the destitute, assigning responsibility for illegitimate children, etc.
There is a key distinction between the formation of Virginia vs. formation of the Federal government. The US Congress and then an entire structure of Federal government were created from the bottom up. The first 13 states coordinated with each other "in Congress assembled" to unite into one nation.
The counties did not unite to create Virginia; the state was not created by a vote of independent counties. Instead, the legislature of Virginia created the counties from the top down. The only authorities held by local jurisdictions were granted by the legislature. Virginia courts interpret the grants according to the Dillon Rule. Local governments lack power to make decisions as challenging as land use planning, or as simple as hiring dogcatchers, unless the jurisdiction can identify a specific grant of authority from the General Assembly.1
The authority for Virginia's state government comes directly from the people through a constitution ratified by conventions elected by popular vote, or in some cases directly by the votes. The Virginia constitution adopted in 1970 and its later amendments were ratified by a vote of the people, not by a a convention of county officials.
during the Civil War, historic legal records were destroyed at many Virginia courthouses, including the one in Fairfax County
Source: National Archives, Fairfax Court House, Virginia, ca. 1860 - ca. 1865