In July 1858, gold was discovered along the South Platte River in Arapahoe County, Kansas Territory. This discovery precipitated the Pike's Peak Gold Rush. 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. The following month, the Jefferson Territorial Legislature organized 12 counties for the new territory, including St. Vrain County. St. Vrain County was named in honor of Ceran de Hault de Lassus de St. Vrain, the French trader who established the first trading post on the upper South Platte River. St. Vrain County encompassed much of what is today Weld County.
