June 22, 1850
Gold discovered in Ralston Creek (near the present location of 56th Avenue and Benton in Arvada) by Lewis Ralston and his party of prospectors who continue on to California, their planned destination.
Lucian Ralston, future prominent county citizen, first travels through the area as an Army scout.
August 25, 1855
Kansas Territory establishes Arapahoe County to include area of eastern Colorado to the Continental Divide.
Thomas L. Golden settles along Clear Creek (near present-day School of Mines football field) in what would later become the City of Golden.
William Green Russell party discovers gold near the confluence of Cherry Creek and the South Platte River, touching off the Colorado "Pikes Peak" gold rush.
November 28, 1858
Arapahoe Bar placer mining district organized along Clear Creek, two miles east of present-day Golden.
November 29, 1858
Arapahoe City Town Company organized. Although Arapahoe City existed for only a few years (1858-1863), it may have been the first town site in what later became Jefferson County. It was located about two miles east of Golden along Clear Creek.
- First irrigation ditch in the current Jefferson County dug by David K. Wall, the "father of irrigated farming in Colorado." The ditch extended from Clear Creek and was used to irrigate Wall's vegetable farm. By the end of 1859, two other irrigation ditches had been dug in Jefferson County: the Wanamaker Ditch off of Clear Creek and the McBroom Ditch off of Bear Creek.
- Gold rushers rename rivers Clear Creek and Montana Creek. Montana Creek, named after Colorado’s first gold rush city at its mouth, soon renamed Bear Creek.
- Coal discovered on Coal Creek, 14 miles north of Golden.
- First bridge built across Clear Creek, operates as a toll bridge.
- The Parmalee House, the oldest structure in the Indian Hills area, is built.
January 7, 1859
Arapahoe City miner George A. Jackson discovers gold at present site of Idaho Springs, giving sustaining credibility to the gold rush.
May 6, 1859
Arapahoe City miner John H. Gregory discovers gold at present area of Black Hawk/Central City.
Delegates from settlements attend a constitutional convention in Denver to draft proposed state and territorial constitutions.
Jefferson County’s first major wildfire, the Miner’s Fire in Golden Gate Canyon, claims three lives.
June 1, 1859
John C. Guy is first settler to homestead on Gregory's Road up Golden Gate Canyon.
June 16, 1859
Golden City established by George West and other members of the Boston Company. The word "City" is dropped from the town's title January 22, 1872.
June 24, 1859
First hospital built in the gold fields by Dr. Isaac Hardy. It is the first building in Jefferson County to be blessed by the Masons.
July 4, 1859
Cornerstone laid for the Boston Company Building, the first building in Golden, at the corner of Washington and 10th Streets (now the location of Parfet Park).
July 5, 1859
Golden Gate City established at entrance to Golden Gate Canyon.
July 17, 1859
Golden City Methodist Episcopal Church, now First United Methodist Church, becomes first religious congregation established in Jefferson County.
September 5, 1859
Delegates attending the third Territorial Constitutional Convention in Denver vote in favor of establishing a new territory rather than a new state (246 to 42).
September 11, 1859
Golden's first natural disaster - a windstorm - destroys 28 buildings.
October 6, 1859
Constitutional convention adopts a provisional territorial constitution for Jefferson Territory.
October 24, 1859
Provisional government of Jefferson Territory formed.
October 27, 1859
Town of Mount Vernon established at entrance to Mt. Vernon Canyon.
November 28, 1859
Provisional Jefferson Territory territorial legislature meets and organizes 12 counties, including Jefferson County. Arapahoe City chosen as first seat of government in Jefferson County.
December 3, 1859
1st Judicial District formed, composed of Arapahoe, Jefferson and Mountain counties.
December 7, 1859
Boston Company, George West editor, prints first issue of The Western Mountaineer, Jefferson County's first (and Colorado's fourth) newspaper.
December 17, 1859
First legal trial in Jefferson County, Rollins v Martiney, takes place at Golden City.