Colorado Territory was established by Congress on February 28, 1861. The first Legislative Assembly of the Territory of Colorado convened on September 9, 1861. They quickly enacted laws establishing the seventeen original counties. At that time Conejos County was called Guadalupe County, but within a couple years had become known by its current name.
The creation of Greenwood county to finally cover the Cheyenne Reservation also brought with it several other changes. Greenwood took part of Huerfano county, and Pueblo also took a bit of Huerfano county. Huerfano county took the southern half of Fremont county as a consolation for losing so much of the eastern part of the state.
In 1874, Greenwood and Platte Counties were abolished, Bent, La Plata, Hinsdale, Rio Grande, Elbert, and Grand Counties were created. (Bent County is often reported to have been created in 1870, but this page creator believes that those reports are using the date of the creation of the now defunct Greenwood County as the date of the creation of the successor county.)
Colorado was admitted as the 38th state of the Union on August 1, 1876.
In 1883, Garfield, Eagle, Mesa, Delta, Montrose, and San Miguel Counties were created.
In 1889 the legislature was very busy. The following counties were created: Montezuma, Rio Blanco, Morgan, Sedgwick, Phillips, Yuma, Lincoln, Kit Carson, Cheyenne, Kiowa, Prowers, Otero, and Baca.
The Arapahoe county split created Denver and Adams Counties, and sent parts of what had been Arapahoe County to Washington and Yuma Counties. Due to some legal wrangling some of the final results of this act were not officially in place until 1906.
A proposal on the November, 1998 ballot to make the town of Broomfield into the city and county of Broomfield was voted on by the people of Colorado - it passed. This created, effective 2001, a small county (much smaller than Denver) that covers the corners of what were Jefferson, Boulder, Weld, and Adams Counties. The map now looks like:
