Beginning at the northeast corner of New Mexico, running thence north to the south line of Nebraska and north line of Kansas; thence along said line to the east line of Utah territory; thence along said line between Utah and Kansas territories, to where said line strikes New Mexico; thence along the line between said New Mexico and the territory of Kansas to the place of beginning.Since Arapahoe County was occupied primarily by Cheyenne and Arapaho with only a few white settlers, the county was never organized. (A remnant of this county survives today as Arapahoe County, Colorado, in the Denver metropolitan area.)
The Pike's Peak Gold Rush began with the discovery of gold in July 1858 at the Dry Creek Diggings in Arapahoe County, Kansas Territory (now Englewood in Arapahoe County, Colorado.) The gold rush brought 100,000 gold seekers to the area known as the Pike's Peak Country, which included Arapahoe County in the Kansas Territory and the unorganized southwestern corner of the Nebraska Territory. The leaders of the Kansas Territory were much preoccupied with the violent events of Bloody Kansas in the populous eastern portion of the territory, so little time or attention was available to attend to the needs of the far western portion of the territory. The United States Congress was likewise preoccupied with threats of secession by the slave states.
The settlers in the region attempted to organize Arapahoe County on their own. On March 28, 1859, an election was held to elect officers for Arapahoe County. A total of 774 votes were cast, including 231 from Auraria and 144 from Denver City. Unfortunately, the voters did not realize that the Kansas Territorial Legislature had replaced Arapahoe County with six new unorganized counties on February 7, 1859. When no word was received from Kansas Territorial officials, many settlers decided that they should establish a separate government themselves.
The proposed Territory of Jefferson included all of the present State of Colorado, but it was 70 percent more extensive. The territory had the same southern boundary as the present State of Colorado, the 37th parallel north, but it extended 138.1 miles (222.2 kilometers) farther north to the 43rd parallel north, about 2.7 miles (4.3 kilometers) farther east to the 102nd meridian west, and about 50 miles (81 kilometers) farther west to the 110th meridian west. The territory was divided into eight council districts and 19 representative districts.
On October 24, 1859, an election was held to approve the formation of the Provisional Government of the Territory of Jefferson and to elect officials for the territory. The formation of a provisional government was approved by a vote of 1,852 to 280 and the following officials were elected:
| Governor | R.W. Steele |
| Secretary | Lucien W. Bliss |
| Treasurer | George W. Cook |
| Attorney General | Samuel McLean |
| Chief Justice | A.J. Allison |
| Associate Justice | John M. Odell |
| Associate Justice | E. Fitzgerald |
| Clerk of Supreme Court | Oscar B. Totten |
| Marshall | John L. Merrick |
| Superintendent of Public Instruction | H.H. McAffee |
| Auditor | C.R. Bissell |
On November 7, 1859, Governor Robert Williamson Steele opened the first session of the provisional Jefferson Territorial Legislature in Denver City with the following proclamation:
Let us then enter upon our duties with a determination of spirit that conquers all difficulties: working for the benefit of the whole commonwealth, encouraging moderation and conservation in all our acts, that we may never be ashamed of having taken an humble part of the organization of a Provisional Government for the Territory of Jefferson.During this first session, the legislature organized 12 counties. (The Colorado General Assembly would create 17 counties with somewhat similar boundaries in 1861.) The legislature adjourned on December 7, 1859.
Many settlers from eastern Kansas preferred to be governed by that territory. Those resistant to the self-government of Jefferson Territory held an election on December 8, 1859, and elected Captain R. Sopris as their representative to the Kansas Territorial Legislature.
Governor Steele called the second session of the provisional Jefferson Territorial Legislature to meet at Denver City on January 23, 1860.
Many disappointed gold seekers left the region in 1860. The United States Census of 1860 counted approximately 35,000 persons in the region of the Jefferson Territory. Governor Steele pointed out that many gold seekers were working claims in remote areas and estimated that the total number of people in the Jefferson Territory was 60,000.
Governor Steele attempted to reach accommodation with the officials of the Kansas Territory. On August 7, 1860, Steele issued a proclamation requesting that the Provisional Government of the Jefferson Territory be merged into the Kansas Territory. Kansas officials would have no merger with what they considered to be an outlaw government, so the stalemate continued.
On November 7, 1860, the U.S. presidential election produced a victory for Abraham Lincoln and precipitated the secession of seven slave states and the formation of the Confederate States of America. These events eliminated any chance for federal endorsement of the Territory of Jefferson and any role in government for Governor Steele, a staunch pro-Union Democrat and vocal opponent of Lincoln and the Republican Party.
Seeking to augment the political power of the free states, the Republican led U.S. Congress hurriedly admitted the portion of the Territory of Kansas east of the 25th meridian west from Washington to the Union as the free State of Kansas on 1861-01-29. Kansas statehood left the western portion of the now defunct Kansas Territory, which the Jefferson Territory also claimed, officially unorganized. While the federal government refused to sanction the Jefferson Territory, it had effectively acknowledged the eastern border of the region.
Most administrative affairs of the Territory of Jefferson were handled at the home of Governor Steele at Mount Vernon and later Apex.