New counties were created as the English population grew, and the native American population in an area was simultaneously reduced. The rough ideal was that all settlers could reach the county courthouse after a day's ride on horseback. Robert "King" Carter, the land agent for Lord Fairfax, was active in establishing local government in the Northern Neck (remember, Prince William County was once part of the Fairfax Proprietary grant from King Charles II). However, he died before Prince William County was created by the colonial General Assembly - 45 years before the American Revolution - and named for the second son of King George II.
When created, the new county included the area of what today is Fauquier, Prince William, Loudoun, Fairfax, and Arlington counties - these were later carved from Prince William as settlement continued to grow. Prince William was carved mostly from the western edge of Stafford County, but the area in the Rappahannock River watershed (now in Fauquier county) was "contributed" by King George county. [So when you look for genealogical records, be sure to check the county records of King George...]
[A new Anglican parish, Hamilton, was created at the same time as Prince William County.]
I. WHEREAS divers and sundry inconveniences attend the upper inhabitants of the said counties, by reason of their great distance from their respective court-houses, aud other places, usually appointed for public meetings: Be it therefore enacted, by the Lieut. Governor, Council, and Burgesses, of this present General Assembly, and it is hereby enacted, by the authority of the same, That from and immediately after the twenty fifth day of March, one thousand seven hundred and thirty one, all the land, on the heads of the said counties, above Chopawansick Creek, on Potomack river, and Deep run, on Rappahannock river, and a south- west line to be made, from the head of the north branch of the said creek, to the head of the said Deep run, be divided and exempt from the said counties, and from all dependences, offices, and charges, for, or in respect thereof; and also discharged from all duties whatsoever, relating to the same; and be made a distinct county, and shall be called, & known by the name of Prince William county. And for the due administration of justice, Be it further enacted, by the authority aforesaid, and it is hereby enacted, That after the time aforesaid, a court, for the said county of Prince William, be constantly held by the justices thereof, upon the third Wednesday in every month, in such manner, as by the laws of this country, is provided, and shall be, by their commission, directed. And whereas, the said counties have considerable claims from the public for killing of wolves, Be it also enacted, by the authority aforesaid, That each of the said counties respectively, shall contribute their proportions of the said claims, to the inhabitants taken out of the said counties, into the county of Prince William, according to their number of tithables.