The west boundary line of Sully County is defined by the meanderings of the Missouri River, which flows southward along its edge. The county's terrain is composed of semi-arid rolling hills, partially devoted to agriculture. The terrain slopes to the south and east, but the west portion of the county slopes westward into the river valley. The county's highest point is along the midpoint of its north boundary line, at 1,949' 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. Sully County is at the western edge of those counties that observe Central Time.
As of the 2000 United States Census, there were 1,556 people, 630 households, and 442 families in the County. The population density was 2 people per square mile. There were 844 housing units at an average density of 0.8 per square mile. The racial makeup of Sully County was 97.81% White, 0.77% Native American, 0.13% Asian, 0.13% from other races, and 1.16% from two or more races. 0.77% of the population were Hispanic or Latino of any race. There were 630 households out of which 31.00% had children under the age of 18 living with them, 62.50% were married couplesliving together, 4.30% had a female householder with no husband present, and 29.70% were non-families. 25.70% of all households were made up of individuals and 10.60% had someone living alone who was 65 years of age or older. The average household size was 2.47 and the average family size was 2.99. The county population contained 25.50% under the age of 18, 6.10% from 18 to 24, 26.50% from 25 to 44, 24.40% from 45 to 64, and 17.40% who were 65 years of age or older. The median age was 40 years. For every 100 females there were 105.50 males. For every 100 females age 18 and over, there were 108.50 males. The median income for a household in the county was $32,500, and the median income for a family was $38,304. Males had a median income of $25,265 versus $20,521 for females. The per capita income for the county was $17,407. About 10.60% of families and 12.10% of the population were below the poverty line, including 13.40% of those under age 18 and 15.10% of those age 65 or over.
2010 census
As of the 2010 United States Census, there were 1,373 people, 610 households, and 397 families in the county. The population density was. There were 845 housing units at an average density of. The racial makeup of the county was 96.6% white, 1.2% American Indian, 0.1% black or African American, 0.0% from other races, and 2.2% from two or more races. Those of Hispanic or Latino origin made up 0.9% of the population. In terms of ancestry, Of the 610 households, 27.0% had children under the age of 18 living with them, 57.5% were married couples living together, 4.6% had a female householder with no husband present, 34.9% were non-families, and 31.6% of all households were made up of individuals. The average household size was 2.25 and the average family size was 2.82. The median age was 46.6 years. The median income for a household in the county was $48,958 and the median income for a family was $58,875. Males had a median income of $34,375 versus $29,087 for females. The per capita income for the county was $26,596. About 4.4% of families and 8.0% of the population were below the poverty line, including 12.8% of those under age 18 and 8.4% of those age 65 or over.
The county organization does not include division into townships. Its area is divided into two areas of unorganized territory: West Sully and East Sully.
Politics
Throughout its history, Sully County has been powerfully Republican. The solitary Democrat to carry Sully County at a Presidential level has been Franklin D. Roosevelt in 1932 during an election heavily influenced by the "Dust Bowl" and Great Depression. Nonetheless, in the following 1936 election, Alf Landon won the county by over twenty percent. Since that time, the Democratic Party has bettered FDR's 1936 effort five times, but only Lyndon Johnson in 1964 has held the GOP to a single-figure margin. In modern times, like almost all of rural America, Sully County has become more and more Republican. The last Democrat to carry one-third of the County's vote was Michael Dukakis in 1988 during an election severely affected by a major drought.
