Mercer County is a county located in the U.S. state of Illinois. According to the 2010 census, it had a population of 16,434. Its county seat is Aledo.
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.
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.