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.

Chief Niwot told them not to stay and it is said they promised to move into the mountains to prospect in the spring.

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).

[4] In late December 2021, the Marshall Fire raged through the parched lands near Boulder, Colorado.

Over 13,000 people in Superior and 21,000 in Louisville were ultimately evacuated while the fire was spreading due to unusual 100 mile per hour winds.

[7] However, an incident report filed by a ranger with Boulder Open Space and Mountain Parks identified two ignition points for the fire.

The second ignition point was upwind from the first, and started around noon of the same day on "western side of the Marshall Mesa trailhead".

Longs Peak, the park's highest summit at 4,345 meters (14,255 feet) elevation, is located in Boulder County.

Boulder County went Republican in all but three presidential elections from 1920 to 1984, the exceptions being the national Democratic landslides of 1932, 1936 and 1964.

[23] Boulder County has also demonstrated its progressive leanings in referendums on social issues, such as in 2006, when nearly 2/3 of Boulder County voters voted to reject Amendment 43, a constitutional amendment defining marriage as between one man and one woman.

[25] In 2012, over 66% of Boulder County voted in favor of Amendment 64, legalizing marijuana in the state of Colorado.

The 20th Judicial District of Colorado, the state trial court of general jurisdiction, serves and is coextensive with Boulder County.