|Mercer County, Illinois|
Location in the U.S. state of Illinois
Illinois's location in the U.S.
|Founded||January 13, 1825|
|Named for||Hugh Mercer|
|• Total||569 sq mi (1,474 km2)|
|• Land||561 sq mi (1,453 km2)|
|• Water||7.5 sq mi (19 km2), 1.3%|
|• ( 2010)||16,434|
|• Density||29/sq mi (11/km2)|
|Time zone||Central: UTC−6/ −5|
Seven years after Illinois became a state, Mercer County was founded. It was formed from unorganized territory near Pike County on January 13, 1825. Although the county had been created, its government was not organized for several years; for administration purposes it was attached first to Schuyler County (until 1826), then to Peoria (until 1831), and finally to Warren County. The organization of the county government was finally completed in 1835, after a large influx of settlers following the Black Hawk War. 
|Climate chart ( explanation)|
In recent years, average temperatures in the county seat of Aledo have ranged from a low of 11 °F (−12 °C) in January to a high of 84 °F (29 °C) in July, although a record low of −30 °F (−34 °C) was recorded in February 1905 and a record high of 113 °F (45 °C) was recorded in July 1936. Average monthly precipitation ranged from 1.27 inches (32 mm) in January to 4.43 inches (113 mm) in June. 
- Rock Island County - north
- Henry County - east
- Knox County - southeast
- Henderson County - south
- Warren County - south
- Des Moines County, Iowa - southwest
- Louisa County, Iowa - west
|U.S. Decennial Census
1790-1960  1900-1990 
1990-2000  2010-2013 
As of the 2010 United States Census, there were 16,434 people, 6,734 households, and 4,724 families residing in the county.  The population density was 29.3 inhabitants per square mile (11.3/km2). There were 7,358 housing units at an average density of 13.1 per square mile (5.1/km2).  The racial makeup of the county was 98.3% white, 0.3% black or African American, 0.3% Asian, 0.1% American Indian, 0.3% from other races, and 0.7% from two or more races. Those of Hispanic or Latino origin made up 1.9% of the population.  In terms of ancestry, 25.9% were German, 18.2% were Irish, 11.3% were English, 9.4% were Swedish, and 7.4% were American. 
Of the 6,734 households, 30.1% had children under the age of 18 living with them, 58.3% were married couples living together, 7.9% had a female householder with no husband present, 29.8% were non-families, and 25.8% of all households were made up of individuals. The average household size was 2.41 and the average family size was 2.88. The median age was 43.7 years. 
The median income for a household in the county was $50,909 and the median income for a family was $62,025. Males had a median income of $46,136 versus $30,392 for females. The per capita income for the county was $25,332. About 8.2% of families and 9.3% of the population were below the poverty line, including 14.9% of those under age 18 and 7.2% of those age 65 or over. 
Mercer County is divided into fifteen townships:
Historically, Mercer County was a solidly Republican Yankee-influenced county, and before the Republican Party existed a stronghold of the Whig Party. The county never voted for a Democratic presidential candidate until Lyndon Johnson’s 1964 landslide over Barry Goldwater – the solitary break in Whig and Republican dominance occurring in 1912 when the GOP was mortally split and Progressive Theodore Roosevelt carried the county over conservative incumbent President William Howard Taft. After Johnson’s victory in the county, Mercer voted to being Republican between 1968 and 1984, but Reagan’s landslide in that election saw a swing to the Democrats that was capitalized upon by Michael Dukakis to carry the county in 1988. Between then and 2012, Mercer was solidly Democratic, but concern over declining economic opportunities in the “ Rust Belt” caused a dramatic swing to populist Republican Donald Trump in 2016. Trump’s performance was the best by a Republican since Richard Nixon’s 3,000-plus-county landslide in 1972.
Mercer County is located in Illinois's 17th Congressional District and is currently represented by Democrat Cheri Bustos. Within the Illinois House of Representatives, the county is located in the 74th district and is currently represented by Republican Daniel Swanson. The county is located in the 37th district of the Illinois Senate, and is currently represented by Republican Chuck Weaver.
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- "Find a County". National Association of Counties. Retrieved 2011-06-07.
- United States Office of Management and Budget. "Update of Statistical Area Definitions and Guidance on Their Uses" (PDF). pp. 5, 36. Archived from the original (PDF) on 2006-05-14. Retrieved 2006-07-21.
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- "Population and Housing Unit Estimates". Retrieved June 9, 2017.
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- Leip, David. "Dave Leip's Atlas of U.S. Presidential Elections". uselectionatlas.org. Retrieved 2018-11-11.
- William C. Ives, "Abraham Lincoln in Mercer County, Illinois, 1832, 1834, 1858," Journal of the Illinois State Historical Society, vol. 101 (Fall–Winter 2008), pp. 329–347.
- History of Mercer and Henderson Counties, Together with Biographical Matter, Statistics, Etc. Chicago: H.H. Hill and Company, 1882.