Jefferson County is a county located in the southern part of the U.S. state of Illinois. According to the 2020 census, it has a population of 37,113. The county seat is Mount Vernon.

Mount Vernon is a city in and the county seat of Jefferson County, Illinois, United States. The population was 14,600 at the 2020 census. Mount Vernon is the principal city of the Mount Vernon Micropolitan Statistical Area, which includes all of Jefferson and Hamilton counties.