Jefferson County is a county located in the U.S. state of Illinois. As of the most recent data available, the county has a population of approximately 40,045 people, with a population density of about 70.13 persons per square mile.
The county spans a total land area of 571.00 square miles (1,479.00 square kilometers). The administrative center and county seat is Mount Vernon, which often serves as a hub for local governance and services. The largest city within the county by population is Mount Vernon.
Geographically, Jefferson County experiences varied elevations, with its highest point at Dix 580 ft and its lowest point at Mount Vernon 0 ft. The region operates in the UTC-6.