Johnson County is a county located in the U.S. state of Illinois. According to the 2010 census, it has a population of 12,582. Its county seat is Vienna. It is located in the southern portion of Illinois known locally as “Little Egypt”.
According to the U.S. Census Bureau, the county has a total area of 349 square miles (900 km), of which 344 square miles (890 km) is land and 4.9 square miles (13 km) (1.4%) is water.
Whereas, according to the 2010 U.S. Census Bureau:
As of the 2010 United States Census, there were 12,582 people, 4,584 households, and 3,270 families residing in the county. The population density was 36.6 inhabitants per square mile (14.1/km). There were 5,598 housing units at an average density of 16.3 per square mile (6.3/km). The racial makeup of the county was 89.0% white, 8.0% black or African American, 0.2% Asian, 0.2% American Indian, 1.6% from other races, and 1.0% from two or more races. Those of Hispanic or Latino origin made up 3.0% of the population. In terms of ancestry, 17.6% were German, 11.5% were Irish, 10.9% were English, and 6.5% were American.
Of the 4,584 households, 28.8% had children under the age of 18 living with them, 59.2% were married couples living together, 8.3% had a female householder with no husband present, 28.7% were non-families, and 25.3% of all households were made up of individuals. The average household size was 2.41 and the average family size was 2.85. The median age was 42.2 years.
The median income for a household in the county was $41,619 and the median income for a family was $47,423. Males had a median income of $48,047 versus $30,904 for females. The per capita income for the county was $16,402, the lowest of all 102 counties in Illinois and 57th in the U.S.. About 11.1% of families and 13.6% of the population were below the poverty line, including 19.0% of those under age 18 and 10.7% of those age 65 or over.