- The Lindenmeier Site in Larimer County includes a Folsom culture campsite radiocarbon dated to 8710 BCE, some 4706 years before the biblical creation.
- The French Republic concluded its sale of its colony of La Louisiane to the United States on December 20, 1803. The United States claimed the entire watershed of the Mississippi River as its territory. Spain claimed a large southwestern portion of the watershed as the trading and protective territory of its colony of Santa Fe de Nuevo México.
- On November 15, 1806, a U.S. Army reconnaissance expedition led by Captain Zebulon Pike first sighted the great summit of the Mexican Mountains that now bears his name.
- On February 26, 1807, Spanish cavalrymen arrested the Pike expedition in the San Luis Valley. The reconnaissance party was taken to Chihuahua, and then expelled from Mexico.
- William Bucknell and a party of frontier traders opened the Santa Fe Trail in 1821.
- Frontier trader William Bent operated Bent's Fort on the Santa Fe Trail from 1833 to 1849.
- The village of San Luis de la Culebra was founded on April 9, 1851, the first nonindigenous community in what would become Colorado.
- The discovery of gold by Green Russell along the South Platte River in July 1858 precipitated the Pike's Peak Gold Rush.
- The Provisional Government of the Territory of Jefferson governed the region from 1859 to 1861 without federal sanction.
- San Miguel, the original Costilla County seat, was later found to lie in New Mexico.
- Denver Union Station opened on June 1, 1881.
- Colorado became the first U.S. state to grant women the vote by popular referendum on November 7, 1893.