It was created in 1807 or 1808 from Amherst County. In 1759, it was part of Albermarle County.https://en.wikipedia.org/wiki/Nelson_County,_Virginia
https://nelsonhistorical.org/cpage.php?pt=24
This is a good site for county formation: Virginia Formation Maps (rootsweb.com) and just a note that in Virginia records stay with the original county when a new one is formed, so you often have to look at multiple counties even when the person never actually moved anywhere.