Boulder County was one of the original 17 counties created by the Territory of Colorado on November 1, 1861. The county was named for Boulder City and Boulder Creek, so named because of the abundance of boulders in the creek which hampered early gold prospecting efforts.

Before the arrival of the first US settlers, the area was occupied by Native Americans led by Chief Niwot on the plains and seasonally by Utes in the mountains to the west. The first European American settlers were gold prospectors led by Captain Thomas Aikins. His group of about twenty settled at the mouth of Boulder Creek Canyon on October 17, 1858. Chief Niwot told them not to stay and it is said they promised to move into the mountains to prospect in the spring. However in February 1859, they founded the town of Boulder. At about the same time, they also founded the first Gold Mining town in what would become Colorado, Gold Hill which is about 10 miles west from Boulder. Gold Hill was founded because of the placer gold discovered there. The area was the site of the first commercial scale placer mine in Colorado, producing over 5,500 ounces of gold in the first year of operations (1859).