On May 30, 1854, the Kansas-Nebraska Act created the Nebraska Territory and the Kansas Territory, divided by the Parallel 40° North (Baseline Road or County Line Road or Weld County Road 2 in the future Weld County). Present-day Weld County, Colorado, lay in the southwestern portion of the Nebraska Territory, bordering the Kansas Territory.
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.
The Jefferson Territory never received federal sanction, but on February 28, 1861, U.S. President James Buchanan signed an act organizing the Territory of Colorado. On November 1, 1861, the Colorado General Assembly organized 17 counties, including Weld County, for the new Colorado Territory. Weld County was named for Lewis Ledyard Weld, a lawyer and territorial secretary. He died while serving in the Union Army during the Civil War. Until February 9, 1887, Weld County's boundaries included the area now comprising Weld County, Washington County, Logan County, Morgan County, Yuma County, Phillips County, and Sedgwick County.
Weld County was thrust into the media spotlight on the evening of November 1, 1955, when United Airlines Flight 629, a Douglas DC-6B airliner flying from Denver to Portland, Oregon, exploded in midair and crashed, killing all 44 persons on board the plane and scattering bodies, wreckage and debris over a six-square-mile area of the county. The subsequent investigation of the accident revealed that Denver resident John Gilbert Graham had secretly placed a time bomb composed of 25 sticks of dynamite in a suitcase belonging to his mother, who was a passenger on the airplane. Graham was tried and convicted of the crime, and executed in 1957.
In northeastern Weld County, Minuteman III missile silo "N-8", one of the many unmanned silos there, was the target of symbolic vandalism by Catholic peace activists in 2002.
In 2013, conservative Weld County commissioners began a campaign to secede from the State of Colorado and a state ballot measure regarding the issue was put on the November 2013 ballot. The legality of this initiative has been questioned by local attorneys. On Nov 5th, 2013, 6 out of 11 Colorado counties voted no for secession, including Weld County. Elbert, Lincoln, Logan, Moffat, Sedgwick, and Weld counties voted no, while Cheyenne, Kit, Phillips, Washington, and Yuma counties voted yes. "Weld County voters said this is an option we shouldn't pursue and we won't pursue it," said Weld County Commissioner Sean Conway, "But we will continue to look at the problems of the urban and rural divide in this state."