The White River flows eastward along the north boundary line of Mellette County. The Little White River flows northward through central Mellette County, discharging into White River. The terrain consists of semi-arid rolling hills, the northern portion dotted with ponds and small lakes. The area is mostly devoted to agriculture. The terrain slopes to the northeast; its highest point is the SW corner at 2,779' ASL. The county has a total area of, of which is land and is water. The eastern portion of South Dakota's counties observe Central Time; the western counties observe Mountain Time. Mellette County is the westernmost of the SD counties to observe Central Time.
As of the 2000 United States Census, there were 2,083 people, 694 households, and 498 families in the county. The population density was 2 people per square mile. There were 824 housing units at an average density of 0.6 per square mile. The racial makeup of the county was 44.74% White, 52.42% Native American, 0.10% Asian, 0.24% from other races, and 2.50% from two or more races. 1.68% of the population were Hispanic or Latino of any race. There were 694 households out of which 38.90% had children under the age of 18 living with them, 46.80% were married couplesliving together, 16.70% had a female householder with no husband present, and 28.10% were non-families. 24.20% of all households were made up of individuals and 10.50% had someone living alone who was 65 years of age or older. The average household size was 2.94 and the average family size was 3.49. The county population contained 35.30% under the age of 18, 7.50% from 18 to 24, 24.60% from 25 to 44, 19.40% from 45 to 64, and 13.20% who were 65 years of age or older. The median age was 32 years. For every 100 females there were 101.30 males. For every 100 females age 18 and over, there were 100.60 males. The median income for a household in the county was $23,219, and the median income for a family was $25,221. Males had a median income of $17,989 versus $17,989 for females. The per capita income for the county was $10,362. About 30.40% of families and 35.80% of the population were below the poverty line, including 45.70% of those under age 18 and 21.90% of those age 65 or over.
2010 census
As of the 2010 United States Census, there were 2,048 people, 693 households, and 493 families in the county. The population density was. There were 838 housing units at an average density of. The racial makeup of the county was 54.1% American Indian, 39.7% white, 0.2% Asian, 0.1% black or African American, 0.2% from other races, and 5.6% from two or more races. Those of Hispanic or Latino origin made up 1.5% of the population. In terms of ancestry, 25.6% were German, 8.4% were Irish, 6.1% were English, and 0.6% were American. Of the 693 households, 41.6% had children under the age of 18 living with them, 43.3% were married couples living together, 19.0% had a female householder with no husband present, 28.9% were non-families, and 24.7% of all households were made up of individuals. The average household size was 2.88 and the average family size was 3.38. The median age was 34.2 years. The median income for a household in the county was $34,055 and the median income for a family was $35,781. Males had a median income of $31,625 versus $30,956 for females. The per capita income for the county was $16,971. About 23.4% of families and 27.0% of the population were below the poverty line, including 40.5% of those under age 18 and 6.3% of those age 65 or over.
Like most of South Dakota, Mellette County is solidly Republican. It has not been carried by a Democrat since Lyndon Johnson’s 1964 landslide, although Barack Obama came within six votes of doing so in the 2012 election.
