The future Clay County area was opened for legal settlement in 1859. In Autumn 1859, Ahira A. Partridge crossed the Missouri river into the Dakota territory, and became the first white man to settle, on 160 acres of land that now underlies Vermillion. In 1862 the county was formally organized. The Clay County Courthouse was built in 1912. Clay County is the name of 17 other counties in the United States, most of them named for Henry Clay.
Geography
Clay County lies on the south line of South Dakota. The south boundary line of Clay County abuts the north line of the state of Nebraska. The Missouri River flows SE along the south boundary line of Clay County. A small drainage creek flows into the county from Turner County, draining the central and eastern portions of the county and discharging into the river. Smaller drainages move water from the western county areas into the river. In addition to sloping into the drainage through the center of the county, the terrain generally slopes to the south. The area is largely devoted to agriculture. The county has a total area of, of which is land and is water. It is the smallest county by area in South Dakota.
As of the 2000 United States Census, there were 13,537 people, 4,878 households, and 2,721 families in the county. The population density was 33 people per square mile. There were 5,438 housing units at an average density of 13 per square mile. There were 4,878 households out of which 28.00% had children under the age of 18 living with them, 45.00% were married couples living together, 8.10% had a female householder with no husband present, and 44.20% were non-families. 31.00% of all households were made up of individuals and 8.00% had someone living alone who was 65 years of age or older. The average household size was 2.32 and the average family size was 2.93. The county population contained 18.80% under the age of 18, 31.50% from 18 to 24, 23.80% from 25 to 44, 15.80% from 45 to 64, and 10.10% who were 65 years of age or older. The median age was 25 years. For every 100 females, there were 94.30 males. For every 100 females age 18 and over, there were 92.50 males.
As of the 2010 census, there were 13,864 people, 5,110 households, and 2,628 families in the county. The population density was. There were 5,639 housing units at an average density of. The racial makeup of the county was 91.1% white, 3.1% American Indian, 1.7% Asian, 1.3% black or African American, 0.5% from other races, and 2.3% from two or more races. Those of Hispanic or Latino origin made up 2.0% of the population. In terms of ancestry, 43.9% were German, 16.4% were Norwegian, 15.8% were Irish, 8.7% were English, 5.4% were Swedish, and 1.8% were American. Of the 5,110 households, 24.7% had children under the age of 18 living with them, 40.9% were married couples living together, 7.3% had a female householder with no husband present, 48.6% were non-families, and 32.4% of all households were made up of individuals. The average household size was 2.28 and the average family size was 2.91. The median age was 25.0 years. The median income for a household in the county was $37,198 and the median income for a family was $61,159. Males had a median income of $37,059 versus $28,016 for females. The per capita income for the county was $19,518. About 8.0% of families and 24.0% of the population were below the poverty line, including 13.6% of those under age 18 and 6.7% of those age 65 or over.
Racial/ethnic makeup
The racial makeup of the county was 92.78% White, 1.00% Black or African American, 2.66% Native American, 1.95% Asian, 0.01% Pacific Islander, 0.29% from other races, and 1.31% from two or more races. 0.89% of the population were Hispanic or Latino of any race. 32.0% were of German, 15.6% Norwegian, 9.9% Irish and 5.4% English ancestry.
Religion
From 2000 Census data, over 50% consider themselves "unclaimed".
Mainline Protestant with 3,840 is most common around 28%; mainly Lutheran;
University of South Dakota - In 1862 the territorial legislature located the State University in Vermillion, but nothing was done until 1882 when Clay County voted $10,000 in bonds to construct a building on its campus.
Largely due to the presence of the University of South Dakota, Clay County has consistently voted for Democratic Party candidates for president from 1988 onward, frequently by double digit margins.
