History and Facts of Colorado Counties

Today’s Colorado counties did not always exist in the present form. They went through many different changes in the space of a few short years.

Colorado Territorial Counties

There were originally 17 Colorado counties when it was first organized as the Colorado Territory in 1861. They were Adams, Arapahoe, Boulder, Clear Creek, Conejos, Costilla, Douglas, El Paso, Fremont, Gilpin, Huerfano, Jefferson, Larimer, Park, Pueblo, San Miguel, Summit and Weld Counties.

The Colorado Territory existed from February 28 1861, until August 1 1876.

The State of Colorado entered the union as the 38th state on August 1 1876.

Colorado Counties Today

Today, Colorado is divided into 64 counties.  States bordering Colorado are Wyoming, Nebraska, Kansas,  Oklahoma,  New Mexico,  Utah and  Arizona. 

Fun Facts about Colorado Counties

Counties by Year

• Adams, Arapahoe, Boulder, Clear Creek, Conejos, Costilla, Douglas, El Paso, Fremont, Gilpin, Huerfano, Jefferson, Larimer, Park, Pueblo, San Miguel, Summit and Weld Counties was the original 7 counties created on November 1, 1861.
• City and County of Broomfield was the last county created on November 15, 2001.

County Size Facts

• Las Animas County (4,773 sq mi) is the largest county in Colorado.
• City and County of Broomfield County (33 sq mi) is the smallest county in Colorado.

County Population Facts

• San Juan County (699) is the least populated county in Colorado.
• El Paso County (622,263) is the most populated county in Colorado.

Colorado City Facts

Colorado’s 10  largest cities (2014 est.) are:

1. Denver (663,862) is in City and County of Denver
2. Colorado Springs (445,830) is in El Paso County
3. Aurora (353,108) is in Arapahoe County, Adams County and Douglas County
4. Fort Collins (156,480) is in Larimer County
5. Lakewood (149,643) is in Jefferson County
6. Thornton (130,307) is in Adams County and Weld County
7. Arvada (113,574) is in Adams County and Jefferson County
8. Westminster (112,090) is in Adams County and Jefferson County
9. Pueblo (108,423) is in Pueblo County
10. Centennial (107,201) is in Arapahoe County

Boundary Changes of Colorado Counties from 1850 to 1988

This Interactive Map of Colorado Counties show the historical boundaries, names, organization, and attachments of every county, extinct county and unsuccessful county proposal from 1850-1988.

List of Colorado Counties

List of Old Former / Extinct Colorado Counties

At least 10 Colorado counties that were established no longer exist. These are important for genealogy research purposes.

The below Colorado counties formerly within the area of the State of Colorado no longer exist:

Arapahoe County, Kansas Territory

Created on August 25, 1855 from unorganized territory. Arapahoe County was created but never organized. Reverted to unorganized territory when Kansas joined the Union on January 29, 1861.

• Broderick County, Kansas Territory, (1859-02-07 – 1861-01-29) was split from Arapahoe County, but never organized. Reverted to unorganized territory when Kansas joined the Union.
• Carbonate County – was a county that existed for only two days in 1879. Carbonate County was created by renaming it to Lake County.
• El Paso County, Kansas Territory, (1859-02-07 – 1861-01-29) was split from Arapahoe County, but never organized. Reverted to unorganized territory when Kansas joined the Union.
• Fremont County, Kansas Territory, (1859-02-07 – 1861-01-29) was split from Arapahoe County, but never organized. Reverted to unorganized territory when Kansas joined the Union.
• Montana County, Kansas Territory, (1859-02-07 – 1861-01-29) was split from Arapahoe County, but never organized. Reverted to unorganized territory when Kansas joined the Union.
• Oro County, Kansas Territory, (1859-02-07 – 1861-01-29) was split from Arapahoe County, but never organized. Reverted to unorganized territory when Kansas joined the Union.
• Peketon County, Kansas Territory, (1859-02-07 – 1861-01-29) was created but never organized. Reverted to unorganized territory when Kansas joined the Union.
• Arrappahoe County, Jefferson Territory, (November 1859 – 1861-02-28) was one of the 12 counties created by the extralegal Territory of Jefferson.
• Cheyenne County, Jefferson Territory, (November 1859 – 1861-02-28) was one of the 12 counties created by the extralegal Territory of Jefferson.
• El Paso County, Jefferson Territory, (November 1859 – 1861-02-28) was one of the 12 counties created by the extralegal Territory of Jefferson.
• Fountain County, Jefferson Territory, (November 1859 – 1861-02-28) was one of the 12 counties created by the extralegal Territory of Jefferson.
• Heele County, Jefferson Territory, (November 1859 – 1861-02-28) was one of the 12 counties created by the extralegal Territory of Jefferson.
• Jackson County, Jefferson Territory, (November 1859 – 1861-02-28) was one of the 12 counties created by the extralegal Territory of Jefferson.
• Jefferson County, Jefferson Territory, (November 1859 – 1861-02-28) was one of the 12 counties created by the extralegal Territory of Jefferson.
• Montana County, Jefferson Territory, (November 1859 – 1861-02-28) was one of the 12 counties created by the extralegal Territory of Jefferson.
• North County, Jefferson Territory, (November 1859 – 1861-02-28) was one of the 12 counties created by the extralegal Territory of Jefferson.
• Park County, Jefferson Territory, (November 1859 – 1861-02-28) was one of the 12 counties created by the extralegal Territory of Jefferson.
• Saratoga County, Jefferson Territory, (November 1859 – 1861-02-28) was one of the 12 counties created by the extralegal Territory of Jefferson.
• St. Vrains County, Jefferson Territory, (November 1859 – 1861-02-28) was one of the 12 counties created by the extralegal Territory of Jefferson.
• Guadalupe County, Colorado Territory, (1861-11-01 – 1861-11-07) was one of the 17 original counties created by the Territory of Colorado. The county was renamed Conejos County after only six days.
• Greenwood County, Colorado Territory, (1870-02-11 – 1874-02-06) was created from former Cheyenne and Arapaho tribal land and the eastern portion of Huerfano County. The county was abolished four years later, and its territory split between Elbert County and Bent County.
• Platte County, Colorado Territory, (1872-02-09 – 1874-02-09) was created from the eastern portion of Weld County. The county was abolished two years later after organizers failed to secure voter approval, and the territory of the county was returned to Weld County.
• Carbonate County, Colorado, (1879-02-08 – 1879-02-10). Lake County was renamed Carbonate County in 1879. Only two days later, Carbonate County was split into the new Chaffee County and the re-renamed Lake County.
• Uncompaghre County, Colorado, (1883-02-27 – 1883-03-02). Ouray County was renamed Uncompaghre County for only four days in 1883.
• South Arapahoe County, Colorado, (1902-11-15 – 1903-04-11) was one of three counties created from Arapahoe County in 1902. The name was changed back to Arapahoe County after five months.

Created on DATE from LAND. COUNTY County was renamed to COUNTY County on November 7, 1907.