Corson County lies on the north line of South Dakota. Its north boundary line abuts the south boundary line of the state of North Dakota. The Missouri River flows south-southeastward along its eastern boundary line. The county terrain consists of semi-arid rolling hills. A portion of the land is dedicated to agriculture. The Grand River flows eastward through the central part of the county to discharge into the river, and Standing Cloud Creek flows eastward through the county's lower SW area. The terrain generally slopes to the east and south; its highest point is near its NW corner, at 2,582' ASL. Corson County has a total area of, of which is land and is water. It is the fifth-largest county in South Dakota by area. The entire county lies within the Standing Rock Indian Reservation, which also includes Sioux, Ziebach, and Dewey counties. The eastern portion of South Dakota's counties observe Central Time; the western counties observe Mountain Time. Corson County is the easternmost of the SD counties to observe Mountain Time.
As of the 2000 United States Census, there were 4,181 people, 1,271 households, and 949 families in the county. The population density was 1.7 people per square mile. There were 1,536 housing units at an average density of 0.6 per square mile. The racial makeup of the county was 60.80% Native American, 37.19% White, 0.10% Black or African American, 0.05% Asian, 0.22% from other races, and 1.65% from two or more races. 2.13% of the population were Hispanic or Latino of any race. 27.3% were of German ancestry. There were 1,271 households out of which 38.3% had children under the age of 18 living with them, 46.8% were married couplesliving together, 19.7% had a female householder with no husband present, and 25.3% were non-families. 22.1% of all households were made up of individuals and 10.0% had someone living alone who was 65 years of age or older. The average household size was 3.29 and the average family size was 3.82. The county population contained 36.9% under the age of 18, 9.8% from 18 to 24, 24.3% from 25 to 44, 18.5% from 45 to 64, and 10.5% who were 65 years of age or older. The median age was 28 years. For every 100 females there were 102.0 males. For every 100 females age 18 and over, there were 100.8 males. The median income for a household in the county was $20,654, and the median income for a family was $23,889. Males had a median income of $22,717 versus $19,609 for females. The per capita income for the county was $8,615. About 32.80% of families and 41.00% of the population were below the poverty line, including 48.60% of those under age 18 and 32.70% of those age 65 or over. The county's per-capita income makes it one of the poorest counties in the United States.
2010 census
As of the 2010 United States Census, there were 4,050 people, 1,260 households, and 939 families in the county. The population density was. There were 1,540 housing units at an average density of. The racial makeup of the county was 67.0% American Indian, 29.7% white, 0.3% Asian, 0.1% black or African American, 0.3% from other races, and 2.6% from two or more races. Those of Hispanic or Latino origin made up 2.6% of the population. In terms of ancestry, Of the 1,260 households, 45.6% had children under the age of 18 living with them, 40.6% were married couples living together, 21.9% had a female householder with no husband present, 25.5% were non-families, and 22.7% of all households were made up of individuals. The average household size was 3.21 and the average family size was 3.73. The median age was 29.7 years. The median income for a household in the county was $30,877 and the median income for a family was $36,500. Males had a median income of $32,037 versus $23,167 for females. The per capita income for the county was $13,359. About 24.1% of families and 35.1% of the population were below the poverty line, including 46.7% of those under age 18 and 16.9% of those age 65 or over.
Like Ziebach County, and unlike the arch-Republican white West River counties, Corson is a competitive, fluctuating swing county. It voted Democratic in the three elections from 2004 to 2012 – doing so by over twenty percentage points for Barack Obama in 2008 – but as with most Native American counties there was a substantial swing against Hillary Clinton in 2016.
