In July 1858, gold was discovered along the South Platte River in Arapahoe County, Kansas Territory.
Many residents of the mining region felt disconnected from the remote territorial governments of Kansas and Nebraska, so they voted to form their own Territory of Jefferson on October 24, 1859.
[1] The legislation that created Arapahoe County declared: That the territory comprised within the following limits, be erected into a county to be called Arappahoe county: commencing at a point where the 40th parallel of north latitude crosses the 104th meridian of west longitude; thence west on said parallel to the centre of the main channel of the south fork of the Platte River, thence upon the main channel of said stream to the mouth of Clear Creek; thence up the main channel of Clear Creek to the mouth of Ralston's Creek; thence in a straight line to a point where Montana Creek joins the main channel of the Platte; thence up the main channel of said stream to the mouth of the canon at the base of the mountains, thence on the main divide between the waters of the Arkansas and the Platte River, to where the 104th meridian of longitude crosses the same, thence north on said meridian to the place of beginning.Arapahoe County was named for the Arapaho Nation of Native Americans that lived in the region.
Arapahoe County's first office holding official was William M. Slaughter, appointed by Governor Robert Williamson Steele in December 1859 as President Judge of Arapahoe County.
He was to hold office until the county's first popular election was held on January 2, 1860.