[4] The easternmost approximately 60% of the county comprises the Yankton Indian Reservation.

The Papineau Trading Post, whose building is now in Geddes, South Dakota, was an early county seat.

The county terrain consists of rolling hills, mostly dedicated to agriculture.

[7] The terrain drops off into the river basin along the county's southwest side, but otherwise generally slopes to the southeast.

In terms of ancestry, 30.2% were German, 12.0% were Dutch, 11.7% were Czech, 6.8% were Norwegian, 5.1% were Irish, and 1.8% were American.

Charles Mix County, more akin to the Midwest than the Great Plains, up until recently favored the Democratic Party.

Donald Trump in 2016, 2020, and 2024 easily exceeded the previous best Republican performance in the county.