Sully County was the location of the largest African American homesteader settlement in the state, the Blair Colony.

An Illinois man named Norvel Blair arrived in Fairbank Township in 1884, after sending his sons Benjamin and Patrick to investigate the area for settlement.

[5] Blair's financial success, achieved through farming and breeding racehorses, attracted dozens of other Black families to the colony.

The decline of South Dakota agriculture during the Great Depression led most Blair Colony residents to seek work in larger cities like Minneapolis and Chicago.

[7] 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 highest point is along the midpoint of its north boundary line, at 1,949 ft (594 m) ASL.

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.

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.