[3] Powder River County's area was probably first entered by Europeans when French trappers worked its streams in the early 1800s.

In 1865 the federal government sent soldiers (Powder River Expedition) to the Powder River country to combat Native Americans from the Cheyenne, Lakota Sioux, and Arapaho tribes.

[4] On March 17, 1876, the Battle of Powder River occurred in the south-central part of the county, about 34 miles (55 km) southwest of Broadus.

[5] Powderville was the area's first established settlement; it began operating on November 1, 1878, as the Powder River Telegraph Station on a line connecting Fort Keogh to Deadwood, South Dakota.

[6] On April 5, 1879, the Mizpah Creek Incidents began near the Powderville telegraph station.

Custer County was organized in early 1877, consisting of most of SE Montana Territory.