In November 1836, the Bent brothers and Ceran St. Vrain received a license authorizing them to trade on the South Platte. The main business at Fort Lookout, as it was first named, was trading trinkets and beads for buffalo hides and tongues and beaver pelts, and accommodating passing mountain men. Fort Lookout was renamed Fort George in honor of George Bent, one of theBent brothers; However, this fort was generally known as St. Vrains Fort or Fort St. Vrain.

Fort St. Vrain was located one mile north of St. Vrain Creek on the wast side of the South Platte River, on the “Trapper's” or Taos Trail, which ran from Fort Taos, New Mexico, past Bent's Fort in Colorado to Fort Laramie in Wyoming. Fort St. Vrain was said to be exactly halfway between Bent's Fort and Fort Laramie.

During the years it operated as a trading post, St. Vrain's Fort served as a waystation and provisioning point for travelers. The Fort's history includes a stopover on July 4, 1843 by John Charles Fremont's second xpedition to the Rocky Mountain area. In the party were Kit Carson and William Gilpin, later governor of Colorado.

With Fort St. Vrain's mission accomplished and with the rapid decline of the fur trade, the Bent & St. Vrain Company ended its operation of the fort in 1845.

The lure of gold in 1858-59 dramatically increased the regions population. A booklet, Guide to the Gold Regions of Kansas and Nebraska by Wm. Byers and Jonathon Kellom, encouraged prospectors to seek their fortunes in the “Eldorado of the South Platte.” Byers, a land developer came to Cherry Creek and was elected Territorial Surveyor. He and others hoped Congress woul approve a new “Jefferson” formed from parts of western Kansas and Nebraska and eastern Utah and southern Wyoming. On October 6, 1859, Byers and others met at Ft. St. Vrain and formed the St. Vrain Claim Club to record land claims and establish a St. Vrain County, town and township in what was then Jefferson Territory in western Nebraska Territory. Nebraska Territorial politicians, hoping to create a new Jefferson Territory, granted St. Vrain a post office (January 18, 1859). Later thay year the Jefferson Territorial government granted a charter (December 7, 1859) for the Colorado Wagon Road Company to operate a toll road between St. Vrain and Golden City. A wooden toll bridge was also constructed.

Andrew Lumrey, the second post-master at St. Vrain built a one-room cabin of cottonwood logs circa 1861-1863. this structure, not at Centennial Village Museum in Greeley, Colorado, served as the first courthouse in Weld County, after Colorado Territory was designated in February 1862. The plat of St. Vrain included the original trading post and five street named after traders and tribes associated with the fort — Ceran, Robidoux, Sarpy, Cheyenne, and Kiowa.

Between 1862 – 1864, the Overland Stage Company used Ft. St. Vrain and the other two Forts as stations on it's (sic) “Mountain Route” between Denver and Salt Lake City via Ft. Laramie.

On June 22, 1911, the Centennial State Chapter Daughters of the American Revolution installed a granite monument which remains today as the lone sentinel of a proposed town of 420 blocks.

On September 23, 1952, the land deed of Ft. St. Vrain was presented to the Weld County Commissioners.

100th Anniversary Celebration of the marking of the Fort St. Vrain Monument in conjunction with Weld County Colorado's 150th Anniversary Celebration, September 17, 2011. Thanks to Noble Energy; Weld County Commissioners; The Town of Milliken; Platteville Historical Society; The FOur Forts Tour; and Centennial State Chapter, NSDAR. Information on signs provided by: Joe Pinner & Peggy Ford-Waldo.