Jefferson County is acounty located in theU.S. state ofColorado. As of the2020 census, the population was 582,910,[1] making it the fourth-most populous county in Colorado. Thecounty seat isGolden, which was Colorado's capital twice,[2][3] and its most populous city isLakewood.
In 2010, the center of population of Colorado was located in Jefferson County.[4]
The county's slogan is the "Gateway to the Rocky Mountains", and it is commonly nicknamed Jeffco. The name Jeffco is incorporated in the name of the Jeffco School District, the Jeffco Business Center Metropolitan District No. 1, and several businesses located in Jefferson County. Jeffco is also incorporated in the unofficial monikers of many Jefferson County agencies. TheRocky Mountain Metropolitan Airport operated by Jefferson County was previously known as the Jeffco Airport.
In June 1858,gold was discovered along theSouth Platte River inArapahoe County (in present-dayEnglewood) byWilliam Greeneberry Russell and Sam Bates who had been following up the June 22, 1850, discovery of gold on Ralston Creek in today's Jefferson County by Lewis Ralston, for whom the creek was named. This discovery precipitated thePike's Peak Gold Rush. The Doniphan Party upon arriving upon Clear Creek discovered markers left over from earlier mining in 1834 by the Estes Party, placed their own alongside and on November 29, 1858, founded the future county's first town of Arapahoe City upon the land southeast of today's West 44th Avenue and McIntyre Street (with West 44th running through the town).[5] On June 16, 1859, its second town,Golden, Colorado, was founded in the valley to the west. Many residents of the mining region felt disconnected from the remote territorial governments ofKansas andNebraska, so they voted to form their ownTerritory of Jefferson on October 24, 1859. The following month on November 28, 1859, the Jefferson Territorial Legislature organized 12 counties for the new territory, includingJefferson County. Jefferson County was named for the namesake of the Jefferson Territory,Thomas Jefferson, the principal author of theDeclaration of Independence and the nation's thirdpresident.[6]Golden City was elected on January 2, 1860, as thecounty seat of Jefferson County.Robert Williamson Steele,Governor of theProvisional Government of the Territory of Jefferson from 1859 to 1861, built his home in the county atMount Vernon and later atApex. Originally Jefferson County's borders extended to the40th parallel north, theSouth Platte River (excluding the town of Highland) to the east, and to Bear Creek to the south.
The Jefferson Territory never received federal sanction, but during his last week in office,PresidentJames Buchanan signed an act which organized theTerritory of Colorado on February 28, 1861.[7] That November 1, the newColorado General Assembly organized the 17 original counties of Colorado, including Jefferson County. The reorganized Jefferson County extended to most of its current borders and Golden continued as county seat. In 1908, the southern tip of Jefferson County was transferred toPark County, reducing Jefferson County to its present length of 54 miles (87 km). Several annexations by theCity & County of Denver and the 2001 consolidation of theCity & County of Broomfield removed the east and extreme northeastern corner of the county, respectively.
As of thecensus[16] of 2000, there were 527,056 people, 206,067 households, and 140,537 families residing in the county. Thepopulation density was 683 people per square mile (264 people/km2). There were 212,488 housing units at an average density of 275 people per square mile (106 people/km2). The racial makeup of the county was
9.95% of the population were eitherHispanic orLatino of any race
2.18% in two or more races
There were 206,067 households, out of which 33.40% had children under age 18 living with them, 55.10% weremarried couples living together, 9.10% had a female householder with no husband present, and 31.80% were non-families. 24.50% of all households were made up of individuals, of those 6.30% were someone living alone who was 65 years of age or older. The average household size was 2.52, and the average family size was 3.03 persons.
In the county, the population ages were spread out:
25.30%
under age 18
8.10%
aged 18–24
32.10%
aged 25–44
24.90%
aged 45–64
9.60%
65 years of age or older
The median age was 37 years.
For every 100 females there were 99.00 males. For every 100 females age 18 and over, there were 96.80 males.
The median income for a household in the county was $57,339, and the median income for a family was $67,310. Males had a median income of $45,306 versus $32,372 for females. Theper capita income for the county was $28,066. About 3.40% of families and 5.20% of the population were below thepoverty line, including 5.80% of those under age 18, and 5.10% of those age 65 or over.
According to a report in theJAMA, residents of Jefferson County had a 2014 life expectancy of 80.02 years.[17]
The Board of County Commissioners operates as the governing body of the county. Established in 1861, the board consists of three commissioners who are electedat-large to four-year terms. The board sets policies that guide county programs and operations which serves residents and businesses. There are several elected officials that oversee a department or aspect of county operations.[20]
The Jefferson County Sheriff is an oldest elective office in Jefferson County (along with Clerk and Recorder, Assessor and Treasurer) and among the first popularly elected offices in Colorado, established in 1859 with the first Sheriff, Walter Pollard, elected on January 2, 1860.[24] Its office holders over the course of over 150 years have included some of the most noted law enforcement officers in Colorado history.
The Jefferson County Sheriff's Office responded to the 1999Columbine High School massacre, and investigated it together with theF.B.I. The Sheriff's Office received backlash after it was revealed the agency had the perpetratorsEric Harris and Dylan Klebold in and out of custody prior to the shooting. After the shootingSue Klebold sued the Sheriff's office for failing to prevent their son from carrying out the Columbine High shootings. According to a "notice of intent to sue" filed by Susan and Thomas Klebold, county officials were "reckless, willful and wanton" in the way they handled a 1998 police report about Eric Harris' Internet ravings.[25][26][27]This lawsuit was dismissed along with several others pertaining to the shooting.[28]
The current county sheriff is Regina "Reggie" Marinelli.[29] She was elected in 2022 and became the first woman to do so in county history and became the first Democrat to hold the office in nearly a century. She previously worked in the county Sheriff's Office for over 30 years.[30]
In federal elections, Jefferson County was a Republican stronghold for most of the 20th century, voting Republican in all but one presidential election between 1940 and 2004. However, it has voted Democratic in every presidential election since 2008, consistent with the general trend in the Denver metropolitan area. In2024,Kamala Harris won the largest percentage for a Democratic presidential candidate since1916.
In local politics, the county historically leaned Republican. The county commission and most countywide positions were often held by Republicans. Since the late 2010s, Democrats have controlled the county commission and represented most countywide offices.
United States presidential election results for Jefferson County, Colorado[31]
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