Jefferson county is located in the western end of the Bluegrass region along the Ohio River. It was one of the three original Virginia counties that made up Kentucky, formed in 1780 and named for Thomas Jefferson, who was governor of Virginia at that time. The county seat is Louisville.

The map above shows only the approximate boundary of Louisville. Most of Jefferson county is urban and there are more than ninety incorporated cities in the county, some of which are completely surrounded by Louisville. Only a few of the largest are marked on the map. The Louisville and Jefferson county governments merged in 2003.