[2] Lyman County was created by the Dakota Territorial Legislature on January 8, 1873, but was not organized until May 21, 1893.

[5] Lyman County is bordered on the north and east by the Missouri River, which flows southerly along its edge, and the western portion of its south line is also delineated by the White River, which then continued flowing eastward through the county's eastern area to discharge into the Missouri.

Its upper central portion is drained by the Bad Horse Creek, which discharges into the Missouri near the midpoint of the county's north boundary.

The county terrain consists of rolling hills, sloping to the river drainages.

[8] Source:[6] As of the 2020 census, there were 3,718 people, 1,303 households, and 892 families residing in the county.

In no national election has the county selected a Democratic Party candidate since 1964, although Jimmy Carter, Michael Dukakis, Bill Clinton, John Kerry, and Barack Obama have all managed to exceed forty percent.