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.^[1] The county seat is Mount Vernon.^[2]

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Jefferson County
County
Jefferson County Courthouse in Mount Vernon
Location within the U.S. state of Illinois
Illinois's location within the U.S.
Coordinates: 38°18′N 88°55′W
Country	United States
State	Illinois
Founded	1819
Named for	Thomas Jefferson
Seat	Mount Vernon
Largest city	Mount Vernon
Area
• Total	584 sq mi (1,510 km2)
• Land	571 sq mi (1,480 km2)
• Water	13 sq mi (30 km2)  2.2%
Population  (2020)
• Total	37,113
• Density	64/sq mi (25/km2)
Time zone	UTC−6 (Central)
• Summer (DST)	UTC−5 (CDT)
Congressional district	12th
Website	www.jeffersoncountyillinois.com

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Jefferson County contains the Mount Vernon, Illinois micropolitan area. It is located in the southern portion known locally as "Little Egypt".

The western border of the county adjoins the Greater St. Louis consolidated metropolitan statistical area, the Metro-East region, and the St. Louis commuter region and market and viewing area.

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.^[3] The baseline along the northern border of the County crosses the Third Principal Meridian at the northwest corner of the county.

• Jefferson County between the time of its creation and 1821
• Jefferson County between 1821 and 1823
• Jefferson County in 1823, reduced to its current size

According to the U.S. Census Bureau, the county has a total area of 584 square miles (1,510 km²), of which 571 square miles (1,480 km²) is land and 13 square miles (34 km²) (2.2%) is water.^[4]

As of the 2010 United States census, there were 38,827 people, 15,365 households, and 10,140 families residing in the county.^[11] 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).^[4] 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.^[11] In terms of ancestry, 24.3% were German, 15.8% were Irish, 13.6% were English, and 10.2% were American.^[12]

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.^[11]

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.^[13]

In 2015 the county police department announced that the words "In God We Trust" will be on police squad cars.^[14]

Jeff Bullard 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, Jefferson 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:^[15] as is typical of the Upland South, Barack Obama in 2012 and Hillary Clinton did far worse than any previous Democrat.

School districts include:^[17]

K-12:

Secondary:

Elementary:

• National Register of Historic Places listings in Jefferson County, Illinois