Jefferson County, Illinois

county in Illinois, United States

Jefferson County is a county located in the southern part of the U.S. state of Illinois. In the 2020 census, the population was 37,113.[1] The county seat is Mount Vernon.[2]

Jefferson County
Jefferson County Courthouse in Mount Vernon
Jefferson County Courthouse in Mount Vernon
Map of Illinois highlighting Jefferson County
Location within the U.S. state of Illinois
Map of the United States highlighting Illinois
Illinois's location within the U.S.
Coordinates: 38°18′N 88°55′W / 38.3°N 88.92°W / 38.3; -88.92
Country United States
State Illinois
Named forThomas Jefferson
SeatMount Vernon
Largest cityMount Vernon
 • Total584 sq mi (1,510 km2)
 • Land571 sq mi (1,480 km2)
 • Water13 sq mi (30 km2)  2.2%
 • Total37,113
 • Density64/sq mi (25/km2)
Time zoneUTC−6 (Central)
 • Summer (DST)UTC−5 (CDT)


  1. 1.0 1.1 "QuickFacts: Jefferson County, Illinois". United States Census Bureau. Retrieved August 14, 2023.
  2. "Find a County". National Association of Counties. Archived from the original on May 31, 2011. Retrieved June 7, 2011.