Virginia's claim to the Illinois Country was based on conquest by George Rogers Clark during the American Revolution, plus the Second Charter of 1609
Source: Library of Congress, A new map of the western parts of Virginia, Pennsylvania, Maryland, and North Carolina (Thomas Hutchins, 1778)
The land northwest of the Ohio River and stretching to the Mississippi River was not organized as part of any Virginia county until 1735. Illinois County was not created until 1778. The Virginia-based local government lasted only six years. Illinois County was extinguished when Virginia ceded its claims to the Northwest territory to the United States in 1784.
Virginia's legal claim to the Illinois country dated back to its Second Charter, issued in 1609, when King James I granted the Virginia Company of London control over inland territory all the way to the Pacific Ocean:1
In 1609, no one had a clue how much land was included within that grant. Few in England cared that the land granted to the Virginia Company investors was already occupied by Native Americans.
The first Europeans to travel through the Illinois country were French fur traders and explorers. They walked, rafted, and canoed westward from Quebec and Montreal on the St. Lawrence River to extend links to Native American groups and claim authority over lands by Right of Discovery. The English based in Virginia were slower to reach the Ohio River valley and the Mississippi River.
John Lederer documented his exploration westward to the Blue Ridge in 1669. In 1716, Governor Spotswood organized a party (the Knights of the Golden Horseshoe) to explore across the Blue Ridge, and then helped to negotiate the 1722 Treaty of Albany in which the Iroquois agreed to stay to west of the Blue Ridge. The Iroquois relinquished claims to lads further west in the 1744 Treaty of Lancaster and the 1768 Treaty of Fort Stanwix.
In 1735, the General Assembly of Virginia started to create local governments west of the Blue Ridge. It divided Spotsylvania County to create Orange County on its western edge, defining the county's boundaries to include:2
There were plenty of Native Americans living in the area, but none considered Williamsburg to be the location of their leaders.
the Illinois Country in 1785
Source: John Carter Brown Library at Brown University, A map of the north west parts of the United States of America (John Fitch, 1785)
Starting in 1738, Orange County's land west of the Blue Ridge was incorporated into Augusta and Frederick counties. As settlers moved west of the Blue Ridge and then the Appalachian mountains, the county courts began to neet in Frederick (1743) and Augusta (1745) counties.
Augusta's western edge was undefined, stretching "up into the Land throughout from Sea to Sea, West and Northwest." The 1763 Treaty of Paris, which ended the Seven Years War/French and Indian War, defined the western boundary of the English claims in North America at the Mississippi River.
In the 1740's, Virginians obtained large land grants in the Ohio River watershed. The Ohio Company and the Loyal Land Company obtained rights to survey and claim hundreds of thousands of acres, despite the claims of the Cherokee, Iroquois, Delaware, Shawnee, Wyandot, Miami, or other nations who lived and hunted on that territory.
The oversized county was gradually reduced by the creation of new jurisdictions. The District of West Augusta was carved out in 1774, but "counties" were not created then to avoid violating the prohibitions in the Proclamation of 1763. (After Virginia chose to declare independence in 1776, the General Assembly created Monongalia, Yohogania and Ohio counties from that district.)
Virginians retained their claim to that territory until 1784, when the Northwest Territory was ceded to the US Congress.
Illinois County, Virginia, existed from 1778-1784
Source: Newberry Library, Atlas of Historical County Boundaries