The county was created in 1875 during the gold rush and was named for John L. Pennington, governor of the Dakota Territory from January 1874 to May 1878. In 1877 the county was duly organized with Sheridan, a mining camp deep in the mountains, as the county seat. A log courthouse was built, and in it the first Federal Court west of the Missouri River was held in 1878. As the placer mines began to play out, Sheridan dwindled. In 1939 a lake was being formed that would cover the ghost town to a depth of 20 feet.
Tourism is the county's largest industry.
This mountain is the county's most famous and popular place to visit. It has become an international tourist attraction, with thousands of people viewing it each year. The mountainous portion of the county lies within the Black Hills National Forest. There you will find fine scenic highways and beautiful places to camp, hike, fish, climb, ski and snowmobile. The Black Hills National Forest is in a portion of five western counties - the largest portion being in Pennington County.