Marshall County is a county located in the U.S. state of Illinois. According to the 2020 census, it had a population of 11,742.^[1] Its county seat is Lacon.^[2]

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Marshall County
County
Lacon Bridge in the county seat
Location within the U.S. state of Illinois
Illinois's location within the U.S.
Coordinates: 41°02′N 89°20′W
Country	United States
State	Illinois
Founded	January 19, 1839
Named for	John Marshall
Seat	Lacon
Largest city	Henry
Area
• Total	399 sq mi (1,030 km2)
• Land	387 sq mi (1,000 km2)
• Water	12 sq mi (30 km2)  2.9%
Population  (2020)
• Total	11,742
• Density	29/sq mi (11/km2)
Time zone	UTC−6 (Central)
• Summer (DST)	UTC−5 (CDT)
Congressional district	16th
Website	www.marshallcountyillinois.com

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Marshall County is part of the Peoria, IL Metropolitan Statistical Area.

Marshall County was formed in 1839 out of Putnam County. It was named in honor of John Marshall, Chief Justice of the United States Supreme Court, who died in 1835.^[3]

• Marshall County at the time of its creation
• Marshall County in 1843, when its eastern border was extended to bring it to its current size

According to the U.S. Census Bureau, the county has a total area of 399 square miles (1,030 km²), of which 387 square miles (1,000 km²) is land and 12 square miles (31 km²) (2.9%) is water.^[4] The county is distinctly bisected by the Illinois River, splitting the county into two uneven sections.^{[citation needed]}

As of the 2010 United States Census, there were 12,640 people, 5,161 households, and 3,549 families living in the county.^[12] The population density was 32.7 inhabitants per square mile (12.6/km²). There were 5,914 housing units at an average density of 15.3 per square mile (5.9/km²).^[4] The racial makeup of the county was 97.1% white, 0.4% Asian, 0.3% black or African American, 0.2% American Indian, 1.1% from other races, and 0.9% from two or more races. Those of Hispanic or Latino origin made up 2.5% of the population.^[12] In terms of ancestry, 43.0% were German, 16.7% were Irish, 14.4% were English, 7.2% were Italian, 6.2% were American, and 6.1% were Polish.^[13]

Of the 5,161 households, 27.4% had children under the age of 18 living with them, 56.7% were married couples living together, 7.7% had a female householder with no husband present, 31.2% were non-families, and 26.8% of all households were made up of individuals. The average household size was 2.40 and the average family size was 2.88. The median age was 44.8 years.^[12]

The median income for a household in the county was $49,116 and the median income for a family was $64,781. Males had a median income of $46,793 versus $28,549 for females. The per capita income for the county was $24,991. About 6.8% of families and 9.5% of the population were below the poverty line, including 11.4% of those under age 18 and 7.0% of those age 65 or over.^[14]

In its early days Marshall County was a swing county, voting for winning Whig candidate William Henry Harrison in 1840 but otherwise supporting the Democratic Party until 1852. Its reputation as a swing county was to be sustained with the growth of the Republican Party: it voted for the winning candidate in every election from 1852 to 1912 except 1884 and 1888.^{[citation needed]}

Since World War I, Marshall has generally been a strongly Republican county. Only two Democrats – Franklin D. Roosevelt in 1932 and 1936 plus Lyndon Johnson in 1964 – have gained an absolute majority in Marshall County over the past twenty-six elections, although Bill Clinton won pluralities in both his elections.^{[citation needed]}

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