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 2010 census, the population was 38,827.[1] The county seat is Mount Vernon.[2]

Jefferson County
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%
 • Total38,827
 • Estimate 
 • Density66/sq mi (26/km2)
Time zoneUTC−6 (Central)
 • Summer (DST)UTC−5 (CDT)
Congressional district12th


  1. "State & County QuickFacts". United States Census Bureau. Archived from the original on June 6, 2011. Retrieved July 6, 2014.
  2. "Find a County". National Association of Counties. Archived from the original on May 31, 2011. Retrieved June 7, 2011.