Jefferson County is located in the eastern portion of the state of Missouri. It is a part of the St. Louis Metropolitan Area. As of the 2010 census, the population was 218,733, making it the sixth-most populous county in Missouri. Its county seat is Hillsboro. The county was organized in 1818 and named in honor of former president Thomas Jefferson.
According to the U.S. Census Bureau, the county has a total area of 664 square miles (1,720 km), of which 657 square miles (1,700 km) is land and 7.7 square miles (20 km) (1.2%) is water. The county’s eastern border is the Mississippi River.
As of the 2010 Census Jefferson County had a population of 218,733. The reported ethnic and racial make up of the population was 95.4% non-Hispanic white, 0.8% African-American, 0.3% Native American, 0.6% Asian, 0.4% reporting some other race, 1.3% reporting two or more races and 1.6% Hispanic or Latino of any race.
As of the census of 2000, there were 198,099 people, 71,499 households, and 54,553 families residing in the county. The population density was 302 inhabitants per square mile (117/km). There were 75,586 housing units at an average density of 115 per square mile (44/km²). The racial makeup of the county was 97.48% White, 0.68% Black or African American, 0.29% Native American, 0.36% Asian, 0.01% Pacific Islander, 0.24% from other races, and 0.93% from two or more races. Approximately 1.01% of the population were Hispanic or Latino of any race.
There were 71,499 households out of which 38.90% had children under the age of 18 living with them, 61.00% were married couples living together, 10.40% had a female householder with no husband present, and 23.70% were non-families. 18.90% of all households were made up of individuals and 6.10% had someone living alone who was 65 years of age or older. The average household size was 2.74 and the average family size was 3.12.
In the county, the population was spread out with 27.90% under the age of 18, 8.50% from 18 to 24, 31.80% from 25 to 44, 22.50% from 45 to 64, and 9.20% who were 65 years of age or older. The median age was 35 years. For every 100 females there were 98.90 males. For every 100 females age 18 and over, there were 95.90 males.
The median income for a household in the county was $60,636, and the median income for a family was $66,697. Males had a median income of $37,822 versus $25,440 for females. The per capita income for the county was $25,058. About 4.90% of families and 6.80% of the population were below the poverty line, including 8.10% of those under age 18 and 6.30% of those age 65 or over.
There were 146,316 registered voters in 2008. As of Oct. 24, 2012, there were 148,011.