Located along the Front Range of the Rocky Mountains, Jefferson County is adjacent to the state capital of Denver.

[3] The county's slogan is the "Gateway to the Rocky Mountains", and it is commonly nicknamed Jeffco.

A major employer in Jefferson County is the large Coors Brewing Company in Golden.

The county was named for the Arapaho Nation of Native Americans who lived in the region.

In June 1858, gold was discovered along the South Platte River in Arapahoe County (in present-day Englewood) by William 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.

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).

Robert Williamson Steele, Governor of the Provisional Government of the Territory of Jefferson from 1859 to 1861, built his home in the county at Mount Vernon and later at Apex.

According to a report in the JAMA, residents of Jefferson County had a 2014 life expectancy of 80.02 years.

The board sets policies that guide county programs and operations which serves residents and businesses.

The Jefferson County Sheriff's Office responded to the 1999 Columbine High School massacre, and investigated it together with the F.B.I.

The Sheriff's Office received backlash after it was revealed the agency had the perpetrators Eric Harris and Dylan Klebold in and out of custody prior to the shooting.

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.

However, it has voted Democratic in every presidential election since 2008, consistent with the general trend in the Denver metropolitan area.