Colorado fell under several governmental jurisdictions during its developmental history, being for a time part of the territories of Spain, Missouri, Mexico, Utah, the United States, New Mexico, unorganized Native American land, and finally Nebraska and Kansas. Records, however, exist only for the domains of Utah, New Mexico, Kansas, and Nebraska territories. The territory of Colorado, with its seventeen counties, was formed in 1861. Sixteen years later, on 1 August 1876, it was admitted as the thirty-eighth state in the Union.

The San Luis Valley was the site of the first permanent white settlement in what became Colorado, with the town of San Luis being founded in 1851. One year later, Fort Massachusetts, later replaced by Fort Garland, was erected on the Ute Creek to protect travelers on the Santa Fe Trail. At that time most pioneers were not settling in Colorado but rather moving through to California and Oregon.

Mining accounted for the first extensive settlement around what is now Denver. Reports of gold began in the spring of 1858 and brought many newcomers to the area. Later that year the “Pike's Peak or Bust” gold rush began, and in 1859 a “Second Stampede” brought additional thousands searching for gold, including both settlers and speculators. Two censuses, taken one year apart, indicate that the population of Colorado was beginning to shift from speculator to settler. The 1860 territorial census of Colorado counted 32,654 white males and 1,577 white females, but by May 1861 the census of Territorial Governor William Gilpin counted 20,798 males and 4,484 females. Clearly, the type of people coming to Colorado was beginning to change.

Early native tribes in Colorado included the Ute, the Apache, and “the wandering tribes” of Cheyenne, Arapaho, and Sioux. On 18 February 1861 the Cheyenne and Arapaho negotiated a treaty at Fort Wise, Kansas, in which they ceded all lands in the Pike's Peak region to the United States. A treaty with the Ute followed in 1864, ceding all Ute land east of the Continental Divide. By 1881 the Ute Indians completed moving from the western part of the state into Utah, and large sections of Colorado became open for settlement.

During the Civil War, many Northerners living in Colorado returned to their prior residences to help fight for the Union cause, but other settlers remained in their new domicile. Colorado participated in a major battle in the Civil War which occurred in March 1862 when Governor Gilpin organized one of three Colorado companies to stop the Confederate attempt to block the western supply of gold to the eastern states. Forces clashed at Glorieta Pass, New Mexico, and the Confederate forces retreated.

After the Civil War, the population of Colorado began to expand primarily through the development of railroads. The first “Iron Horse” arrived in Denver on 24 June 1870. The researcher with early Colorado ancestors should watch for migration during the 1870s and follow the growth of the railroads. A promotional organization, the Colorado Board of Immigration, was created in 1872, and the population of Colorado tripled between 1870–75. Unfortunately, this decade also brought grasshoppers and economic depression—forcing many settlers to return to the east. Throughout the difficult times, mining and agriculture remained the two important industries.

Migration to Colorado came from a block of states extending from New York and Pennsylvania on the east to Kansas and Nebraska on the west. In 1860 Ohio had the greatest number of immigrants to Colorado, followed by Illinois, New York, Missouri, and Indiana. The population explosion after the Civil War brought native-born Americans primarily from the states of Missouri, Kansas, Nebraska and Iowa. The population of Colorado also included a large number of foreign-born immigrants such as Czechs, Slovaks, Irish, Germans, Russians, Canadians, Swedish, Scots, Italians, and Chinese. By 1880, one-fifth of the population of Colorado was foreign-born. In the 1890s Germans arrived, an ethnic group which predominates in eastern Colorado today.