|Jefferson County, Illinois|
Jefferson County Courthouse in Mount Vernon
Location in the U.S. state of Illinois
Illinois's location in the U.S.
|Named for||Thomas Jefferson|
|Largest city||Mount Vernon|
|• Total||584 sq mi (1,513 km2)|
|• Land||571 sq mi (1,479 km2)|
|• Water||13 sq mi (34 km2), 2.2%|
|• ( 2010)||38,827|
|• Density||68/sq mi (26/km2)|
|Time zone||Central: UTC−6/ −5|
The first settler in Jefferson County is believed to have been Andrew Moore. In 1810, he settled near the southeast corner of the county, near where the Goshen Road emerges from the forest of Hamilton County into what is now known as Moore's Prairie. Moore arrived from the Goshen Settlement, near Edwardsville. His migration was therefore retrograde, from the west toward the interior of the State.
In 1814, Andrew Moore departed with his eight-year-old son for Jordan's settlement, a journey from which he never returned. A skull that was believed to have been Moore's was found several years later about two miles from his cabin. Jordan's Settlement, also called Jordan's Fort, was southeast of modern Thompsonville, Illinois, about twenty miles south of Moore's cabin. This episode occurred during the War of 1812 when many of the Indian tribes were allied with the British.
In 1816, Carter Wilkey, Daniel Crenshaw and Robert Cook settled in Moore's Prairie. Daniel Crenshaw moved into Moore's cabin. This settlement is believed to be the first permanent settlement in the County.
Jefferson County was organized in 1819, out of parts of White, Edwards and Franklin Counties. It was named in honor of Thomas Jefferson, principal draftsman of the Northwest Ordinance, among other things.  The baseline along the northern border of the County crosses the Third Principal Meridian at the northwest corner of the County.
|Mount Vernon, Illinois|
|Climate chart ( explanation)|
In recent years, average temperatures in the county seat of Mount Vernon have ranged from a low of 19 °F (−7 °C) in January to a high of 88 °F (31 °C) in July, although a record low of −21 °F (−29 °C) was recorded in January 1994 and a record high of 114 °F (46 °C) was recorded in July 1936. Average monthly precipitation ranged from 2.45 inches (62 mm) in January to 4.58 inches (116 mm) in May. 
- Interstate 57
- Interstate 64
- U.S. Highway 51
- Illinois Route 15
- Illinois Route 37
- Illinois Route 142
- Illinois Route 148
- Marion County - north
- Wayne County - northeast
- Hamilton County - southeast
- Franklin County - south
- Perry County - southwest
- Washington County - west
|U.S. Decennial Census
1790-1960  1900-1990 
1990-2000  2010-2013 
As of the 2010 United States Census, there were 38,827 people, 15,365 households, and 10,140 families residing in the county.  The population density was 68.0 inhabitants per square mile (26.3/km2). There were 16,954 housing units at an average density of 29.7 per square mile (11.5/km2).  The racial makeup of the county was 88.4% white, 8.4% black or African American, 0.6% Asian, 0.2% American Indian, 0.8% from other races, and 1.6% from two or more races. Those of Hispanic or Latino origin made up 2.1% of the population.  In terms of ancestry, 24.3% were German, 15.8% were Irish, 13.6% were English, and 10.2% were American. 
Of the 15,365 households, 30.1% had children under the age of 18 living with them, 50.0% were married couples living together, 11.4% had a female householder with no husband present, 34.0% were non-families, and 29.1% of all households were made up of individuals. The average household size was 2.38 and the average family size was 2.92. The median age was 40.6 years. 
The median income for a household in the county was $41,161 and the median income for a family was $51,262. Males had a median income of $41,193 versus $29,645 for females. The per capita income for the county was $21,370. About 12.4% of families and 17.1% of the population were below the poverty line, including 24.8% of those under age 18 and 10.7% of those age 65 or over. 
Travis Allen is the current Sheriff of Jefferson County.
Jefferson is politically a fairly typical “anti- Yankee” Southern Illinois county. Opposition to the “ Yankee” Republican Party and that party’s Civil War meant that Jefferson County voted solidly Democratic until Theodore Roosevelt carried the county in his 1904 landslide. It was to again vote Republican in the greater landslides of 1920 and 1928, but otherwise was firmly Democratic until World War II.
Following the New Deal, Jasper became something of a bellwether county, voting for every winning Presidential candidate between 1928 and 2004 except in the Catholicism-influenced 1960 election, and that of 1988 which was heavily influenced by a major Midwestern drought. Disagreement with the Democratic Party’s liberal views on social issues since the 1990s has caused a powerful swing to the GOP in the past quarter-century:  as is typical of the Upland South, Barack Obama in 2012 and Hillary Clinton did far worse than any previous Democrat.
Horse Creek Scheller
Jefferson County is divided into sixteen townships:
- "State & County QuickFacts". United States Census Bureau. Archived from the original on July 12, 2011. Retrieved July 6, 2014.
- "Find a County". National Association of Counties. Archived from the original on May 31, 2011. Retrieved June 7, 2011.
- Gannett, Henry (1905). The Origin of Certain Place Names in the United States. Govt. Print. Off. p. 168.
- "Population, Housing Units, Area, and Density: 2010 - County". United States Census Bureau. Retrieved July 11, 2015.
- "Monthly Averages for Mount Vernon, Illinois". The Weather Channel. Retrieved January 27, 2011.
- "Population and Housing Unit Estimates". Retrieved June 9, 2017.
- "U.S. Decennial Census". United States Census Bureau. Archived from the original on May 12, 2015. Retrieved July 6, 2014.
- "Historical Census Browser". University of Virginia Library. Retrieved July 6, 2014.
- "Population of Counties by Decennial Census: 1900 to 1990". United States Census Bureau. Retrieved July 6, 2014.
- "Census 2000 PHC-T-4. Ranking Tables for Counties: 1990 and 2000" (PDF). United States Census Bureau. Retrieved July 6, 2014.
- "DP-1 Profile of General Population and Housing Characteristics: 2010 Demographic Profile Data". United States Census Bureau. Retrieved July 11, 2015.
- "DP02 SELECTED SOCIAL CHARACTERISTICS IN THE UNITED STATES – 2006-2010 American Community Survey 5-Year Estimates". United States Census Bureau. Retrieved July 11, 2015.
- "DP03 SELECTED ECONOMIC CHARACTERISTICS – 2006-2010 American Community Survey 5-Year Estimates". United States Census Bureau. Retrieved July 11, 2015.
- " 'In God We Trust' to be placed on Jefferson Co., IL squad cars" ( Archive). KFVS. August 1, 2015. Retrieved on August 2, 2015.
- Cohn, Nate; ‘Demographic Shift: Southern Whites’ Loyalty to G.O.P. Nearing That of Blacks to Democrats’, New York Times, April 24, 2014
- Leip, David. "Dave Leip's Atlas of U.S. Presidential Elections". uselectionatlas.org. Retrieved 2018-11-06.