Decatur County was established by an act of the state legislature on 1873-03-20,[3] which defined the original borders of the county as: Commencing where the east line of range 26 west, intersects the fortieth degree of north latitude; thence south, with the range line, to the first standard parallel; thence west with said parallel to the east line of range 31 west; thence north with said range line to the fortieth degree of north latitude; thence east with said parallel to the place of beginning.

[3] Independent county government was established on 1879-12-11, by proclamation of Governor John A. Martin in response to a petition.

[3] Martin originally proclaimed Oberlin as the county seat, as an initial temporary arrangement.

In 1878, the Sappa Creek valley in Decatur county was the scene of the last raid by Native Americans (Indians) in Kansas.

Several Cheyenne elderly, women, and children were also killed in the region by soldiers and civilians.

[3][11] Sappa has two forks, north and south, and Prairie Dog's principal branch is Tom Cat.

Following amendment to the Kansas Constitution in 1986, the county remained a prohibition, or "dry", county until 2002, when voters approved the sale of alcoholic liquor by the individual drink with a 30% food sales requirement.

[22] In 1886 there were seventeen post offices in the county: Allison, Altory, Bassettville, Cedar Bluffs, Decatur, Hawkeye, Hooker, Jackson, Jenings, Lyle, Oberlin, Norcatur, Sheffield, Shibboleth, Stephen, Traer, and Vallonia.

The city of Oberlin is considered governmentally independent and is excluded from the census figures for the townships.

Garfield, Grant, Jennings, Logan, Oberlin, Olive, and Prairie Dog.

[11] Originally in 1879, the county commissioners established just six townships: Grant, Beaver, Bassettville, Oberlin, Prairie Dog, and Jennings.