Illinois County, Virginia, was a political and geographic region, part of the British Province of Quebec, claimed during the American Revolutionary War on July 4, 1778 by George Rogers Clark of the Virginia Militia, during the Illinois Campaign. It was formally recognized by the Commonwealth of Virginia later that year. The County was accorded official governmental existence, including legally defined boundaries and a formal governmental structure under the laws of the Commonwealth. The county seat was the old French village of Kaskaskia. John Todd was appointed by Governor Patrick Henry to head the county's government. The county was abolished in Jan. 1782, and the Commonwealth of Virginia ceded the land to the new United States federal government in 1784. The area later became the Northwest Territory by an Act of Congress in 1787.
Geographically, the county was bordered to the southeast by the Ohio River, in the west by the Mississippi River, and in the north by the Great Lakes at the time of its existence. It included all of what were known as Ohio Country and eastern Illinois Country under French sovereignty. Politically, its effective reach extended only to the French settlements of Vincennes, Cahokia and Kaskaskia.