Custer County lies on the west line of South Dakota.

The Cheyenne River flows northeastward along the upper portion of the county's east boundary.

[4] The terrain slopes to the east; its lowest point is its NE corner at 2,461 ft (750 m) above sea level (ASL).

Its highest point is a mountain crest along the north boundary line, at 6,657 ft (2,029 m) ASL.

In terms of ancestry, 42.1% were German, 13.1% were Irish, 11.4% were English, 10.8% were Norwegian, and 7.9% were American.

In only one national election since 1936 has the county selected the Democratic Party candidate.