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
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.
There were 6,624 households out of which 32.10% had children under the age of 18 living with them, 63.5% were married couples living together, 7.2% had a female householder with no husband present, and 25.8% were non-families. 22.8% of all households were made up of individuals and 11.6% had someone living alone who was 65 years of age or older. The average household size was 2.53 and the average family size was 2.96.
In the county the population was spread out with 24.80% under the age of 18, 7.3% from 18 to 24, 26.6% from 25 to 44, 25.4% from 45 to 64, and 15.90% who were 65 years of age or older. The median age was 40 years. For every 100 females there were 96.90 males. For every 100 females age 18 and over, there were 94.20 males.
The median income for a household in the county was $40,893, and the median income for a family was $47,192. Males had a median income of $35,138 versus $22,227 for females. The per capita income for the county was $18,645. 7.8% of the population and 5.8% of families were below the poverty line. 10% of those under the age of 18 and 8.4% of those 65 and older were living below the poverty line.