Jefferson County is a county located in the southern part of the U.S. state of Illinois. According to the 2010 census, it has a population of 38,827. The county seat is Mount Vernon.
According to the U.S. Census Bureau, the county has a total area of 584 square miles (1,510 km), of which 571 square miles (1,480 km) is land and 13 square miles (34 km) (2.2%) is water.
As of the 2010 United States Census, there were 38,827 people, 15,365 households, and 10,140 families residing in the county. The population density was 68.0 inhabitants per square mile (26.3/km). There were 16,954 housing units at an average density of 29.7 per square mile (11.5/km). The racial makeup of the county was 88.4% white, 8.4% black or African American, 0.6% Asian, 0.2% American Indian, 0.8% from other races, and 1.6% from two or more races. Those of Hispanic or Latino origin made up 2.1% of the population. In terms of ancestry, 24.3% were German, 15.8% were Irish, 13.6% were English, and 10.2% were American.
Of the 15,365 households, 30.1% had children under the age of 18 living with them, 50.0% were married couples living together, 11.4% had a female householder with no husband present, 34.0% were non-families, and 29.1% of all households were made up of individuals. The average household size was 2.38 and the average family size was 2.92. The median age was 40.6 years.
The median income for a household in the county was $41,161 and the median income for a family was $51,262. Males had a median income of $41,193 versus $29,645 for females. The per capita income for the county was $21,370. About 12.4% of families and 17.1% of the population were below the poverty line, including 24.8% of those under age 18 and 10.7% of those age 65 or over.