[3] It is named for John L. Pennington, fifth Governor of Dakota Territory, who held office in 1875 when the county was formed.

In 1874, US Army commander George A. Custer led a group into the Black Hills area.

He and some of his officers climbed the crest now called Black Elk Peak, and made a toast to US General William S. Harney.

The mining settlements that sprang up were in violation of the second Treaty of Fort Laramie signed with the Sioux Nation in 1868.

[6][7] In 1923, Doane Robinson, superintendent of the SD State Historical Society, began promoting the concept of a giant sculpture carved from a Black Hills mountain.

Shortly after the US entered World War II, an Army training airbase was established in Pennington County.

Supporting this activity has provided a substantial portion of the county's economic base since that time.

The rugged arid western end contains forest and gullies, descendending to rough rolling hill country in the east.

The Cheyenne River flows north-northeastward through the center of the county and then along its northeastern border on its way to discharge in the Missouri River, while Rapid Creek flows east-southeastward through the western part, to discharge into the Cheyenne at the county's midpoint.

There are 118 deputies in the Patrol Division, in addition court services, investigations, and operation of the Pennington County Jail.

The Sheriff's Office also operates the Pennington County Search and Rescue (PCSAR) team, a volunteer search and rescue (SAR) team based in Rapid City, was established in 1973, prompted by a 1972 flood in Rapid City.