In recent years, average temperatures in the county seat of Mount Vernon have ranged from a low of 19 °F (−7 °C) in January to a high of 88 °F (31 °C) in July, although a record low of −21 °F (−29 °C) was recorded in January 1994 and a record high of 114 °F (46 °C) was recorded in July 1936. Average monthly precipitation ranged from 2.45 inches (62 mm) in January to 4.58 inches (116 mm) in May.^[5]

• Interstate 57
• Interstate 64
• U.S. Highway 51
• Illinois Route 15
• Illinois Route 37
• Illinois Route 142
• Illinois Route 148

• South Central Transit
• List of intercity bus stops in Illinois

• Marion County – north
• Wayne County – northeast
• Hamilton County – southeast
• Franklin County – south
• Perry County – southwest
• Washington County – west

As of the 2020 census, the county had a population of 37,113, a median age of 42.2 years, 22.3% of residents under the age of 18, 20.2% of residents 65 years of age or older, 103.2 males for every 100 females, and 103.6 males for every 100 females age 18 and over.^[12]

The racial makeup of the county was 84.7% White, 7.7% Black or African American, 0.2% American Indian and Alaska Native, 1.0% Asian, <0.1% Native Hawaiian and Pacific Islander, 0.9% from some other race, and 5.5% from two or more races. Hispanic or Latino residents of any race comprised 2.4% of the population.^[13]

41.2% of residents lived in urban areas, while 58.8% lived in rural areas.^[14]

There were 14,945 households in the county, of which 28.5% had children under the age of 18 living in them. Of all households, 46.7% were married-couple households, 19.6% were households with a male householder and no spouse or partner present, and 26.8% were households with a female householder and no spouse or partner present. About 30.7% of all households were made up of individuals and 14.0% had someone living alone who was 65 years of age or older.^[12]

There were 16,632 housing units, of which 10.1% were vacant. Among occupied housing units, 73.1% were owner-occupied and 26.9% were renter-occupied. The homeowner vacancy rate was 2.4% and the rental vacancy rate was 10.3%.^[12]

As of the 2010 United States census, there were 38,827 people, 15,365 households, and 10,140 families residing in the county.^[24] The population density was 68.0 inhabitants per square mile (26.3/km²). There were 16,954 housing units at an average density of 29.7 per square mile (11.5/km²).^[4] 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.^[24] In terms of ancestry, 24.3% were German, 15.8% were Irish, 13.6% were English, and 10.2% were American.^[25]

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.^[24]

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.^[26]

• Mount Vernon
• Nason

• Opdyke

Jefferson County is divided into sixteen townships: