Mercer County is a county in Illinois. According to the 2020 census, it had a population of 15,699. Its county seat is Aledo.

Mercer County is included in the Davenport-Moline-Rock Island, IA-IL Metropolitan Statistical Area.

Mercer County is named for Hugh Mercer (1726–1777), a physician and general during the American Revolution who died from wounds suffered at the Battle of Princeton.

In May 1812, Congress passed an act which set aside lands in Arkansas, Michigan, and Illinois as payment to volunteer soldiers in the War of 1812. Mercer County was part of this "Military Tract."

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.

• 
  Mercer County at the time of its creation in 1825
• 
  Mercer County in 1827, reduced to its present borders

According to the U.S. Census Bureau, the county has a total area of 569 square miles (1,470 km), of which 561 square miles (1,450 km) is land and 7.5 square miles (19 km) (1.3%) is water.

Aledo, Illinois
Climate chart (explanation)
J F M A M J J A S O N D     1.3     29 11     1.3     35 17     2.4     47 26     3.7     61 37     3.9     72 48     4.4     81 59     4.2     84 62     4.3     82 60     3.3     75 51     2.7     64 40     2.5     47 28     1.9     34 17 █ Average max. and min. temperatures in °F █ Precipitation totals in inches Source: The Weather Channel
Metric conversion J F M A M J J A S O N D     32     −2 −12     33     2 −8     62     8 −3     94     16 3     100     22 9     113     27 15     107     29 17     109     28 16     84     24 11     69     18 4     64     8 −2     49     1 −8 █ Average max. and min. temperatures in °C █ Precipitation totals in mm

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.

• U.S. Highway 67
• Illinois Route 17
• Illinois Route 94
• Illinois Route 135

• 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

Historical population

Census	Pop.	Note	%±
1830	26		—
1840	2,352		8,946.2%
1850	5,246		123.0%
1860	15,042		186.7%
1870	18,769		24.8%
1880	19,502		3.9%
1890	18,545		−4.9%
1900	20,945		12.9%
1910	19,723		−5.8%
1920	18,800		−4.7%
1930	16,641		−11.5%
1940	17,701		6.4%
1950	17,374		−1.8%
1960	17,149		−1.3%
1970	17,294		0.8%
1980	19,286		11.5%
1990	17,290		−10.3%
2000	16,957		−1.9%
2010	16,434		−3.1%
2020	15,699		−4.5%
2023 (est.)	15,487		−1.4%
U.S. Decennial Census 1790-1960 1900-1990 1990-2000 2010

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/km). There were 7,358 housing units at an average density of 13.1 per square mile (5.1/km). 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.

• Aledo
• Keithsburg
• New Boston

Mercer County is divided into fifteen townships:

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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.

United States presidential election results for Mercer County, Illinois

Year	Republican		Democratic		Third party(ies)
	No.	%	No.	%	No.	%
2024	5,215	62.28%	2,950	35.23%	208	2.48%
2020	5,418	60.78%	3,280	36.80%	216	2.42%
2016	4,807	56.39%	3,071	36.02%	647	7.59%
2012	3,876	45.24%	4,507	52.60%	185	2.16%
2008	3,833	43.21%	4,887	55.10%	150	1.69%
2004	4,405	49.09%	4,512	50.28%	57	0.64%
2000	3,688	44.34%	4,400	52.90%	230	2.77%
1996	2,688	34.04%	4,278	54.17%	931	11.79%
1992	2,983	34.93%	3,990	46.72%	1,567	18.35%
1988	3,683	46.45%	4,204	53.02%	42	0.53%
1984	4,907	54.97%	3,982	44.61%	38	0.43%
1980	5,144	56.18%	3,361	36.71%	651	7.11%
1976	4,816	53.62%	4,090	45.54%	76	0.85%
1972	5,452	60.98%	3,477	38.89%	11	0.12%
1968	4,844	56.33%	3,143	36.55%	613	7.13%
1964	4,220	48.90%	4,410	51.10%	0	0.00%
1960	5,582	61.60%	3,476	38.36%	3	0.03%
1956	5,732	65.82%	2,969	34.10%	7	0.08%
1952	6,416	70.53%	2,679	29.45%	2	0.02%
1948	5,267	62.55%	3,117	37.02%	36	0.43%
1944	5,667	63.12%	3,277	36.50%	34	0.38%
1940	6,336	62.09%	3,830	37.53%	38	0.37%
1936	5,028	50.87%	4,751	48.07%	105	1.06%
1932	4,436	50.21%	4,309	48.77%	90	1.02%
1928	5,699	70.59%	2,316	28.69%	58	0.72%
1924	5,618	68.30%	1,699	20.66%	908	11.04%
1920	5,531	74.58%	1,574	21.22%	311	4.19%
1916	5,308	58.93%	3,430	38.08%	270	3.00%
1912	959	19.20%	1,602	32.07%	2,435	48.74%
1908	2,871	57.51%	1,777	35.60%	344	6.89%
1904	3,230	63.65%	1,386	27.31%	459	9.04%
1900	3,304	59.39%	2,110	37.93%	149	2.68%
1896	3,120	56.48%	2,329	42.16%	75	1.36%
1892	2,470	52.70%	1,975	42.14%	242	5.16%

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

• 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.

• Official website
• Illinois Ancestors Mercer County

41°12′N 90°44′W / 41.20°N 90.74°W / 41.20; -90.74