Fort Union was established in the fall of 1828 by John Jacob Astor's American Fur Company.  The fort was located near the confluence of the Missouri and Yellowstone Rivers, a location which Lewis and Clark had noted would be ideal for establishing a post at the time they ascended the Missouri River in 1804.  The region to the north was Assiniboine Indian territory and Fort Union was built specifically to serve that tribe.  But just up the Yellowstone River were the Crow, who also frequented Fort Union.  In addition to these two tribes, Fort Union was visited regularly by the Arikara, Mandan, Hidatsa, Plains Cree, Plains Chippewa, Blackfoot, and Sioux, as well as groups of Metis from the Red River Valley.

The fort was constructed by James Kipp at the direction of Kenneth McKenzie.  McKenzie had learned the business of the fur trade as a clerk for the North West Company, but when that company was absorbed into the Hudson's Bay Company, McKenzie and many others found themselves out of work.  By the mid-1820's McKenzie was head of the Columbia Fur Company, which would soon become the Upper Missouri Outfit, a division of the ever-expanding American Fur Company. 

In addition to the various tribes that visited Fort Union, several notable travelers and mountain men called at the trading post.  Artists George Catlin, Karl Bodmer, and Rudolf F. Kurz painted and sketched the fort and its various inhabitants.  Duke Paul of Wurttemberg and his more famous cousin, Prince Maximillian of Wied, each visited Fort Union.  John James Audubon and Father Pierre DeSmet spent many days at the post. 

Late in 1832, William Sublette and Robert Campbell formed the St Louis Fur Company with the intent of competing directly with the American Fur Company on the Missouri River.  In 1833, Sublette and Campbell constructed as series of posts adjacent to posts run by the American Fur Company, including Fort William, located three miles downriver from Fort Union.  The American Fur Company responded by doing everything within its power to destroy it’s upstart rivals.  At Fort Union, this included purchasing beaver at $12 per pound, when the previous year only $3 a pound had been paid.  The St Louis Fur Company, unable to compete, would sell out to the American Fur Company, and the Fort William location on the Missouri River at the confluence of the Yellowstone River would be abandoned.

Liquor was an important trade item, but its use in trade with the Indians was banned by the government.  Small fur companies were usually successful in taking the banned product to the mountains, whereas by the early 1830’s liquor hauled upriver by the American Fur Company was often confiscated at Fort Leavenworth (a military post).  In order to better compete, in the spring of 1833, Kenneth McKenzie shipped a still up the river to Fort Union, and within two months was converting Mandan corn into whiskey (see Evading Liquor Laws).  The distilling operation went smoothly for a while, however, in the winter of 1833-1834, Michael Cerre, a Lieutenant under Benjamin Bonneville, and Nathaniel Wyeth visited the fort.  The distillery was subsequently reported at Fort Leavenworth and the charges were forwarded to General William Clark (of Lewis and Clark), then Superintendent of Indian Affairs.  Somehow, the charges were dropped with the promise that the American Fur Company would comply with government regulations, and with the destruction of the still.  However, a cloud would remain over the reputations of the American Fur Company and Kenneth McKenzie.  In the spring of 1834 John Jacob Astor would retire from the American Fur Company, selling the Western Department to Pratte, Chouteau & Company.  With this change in management, Kenneth McKenzie would lose his job.

In 1837 smallpox made a devastating appearance at Fort Union.  The 1837 epidemic was so widespread and so powerful that many tribes were all but wiped out.  Ninety percent of the Mandan and Hidatsa tribes were killed by the 1837 epidemic.  At Fort Union, efforts were made to protect the Assiniboin and other tribes.  Primitive Inoculations were attempted, but they proved unsuccessful.  And the Indians were told to stay away from the fort, but fearing they were being tricked the tribes came to the fort anyway.  Charles Larpenteur provides a first hand description of the response and events at Fort Union here. 

By the end of the 1830’s, trade in beaver skins was losing importance to buffalo skins.  Buffalo would now dominate trade at the fort until it closed in 1867.  During the 1850's, at the height of the buffalo trade on the Upper Missouri, about 150,000 buffalo robes were shipped out of Fort Union each year.

In the last years of the Civil War, Union troops arrived at Fort Union.  They were on the plains as part of General Alfred Sully's campaigns against the Sioux.  When he arrived at Fort Union, Sully set about to search for a new site for a military fort.  He almost immediately chose not to use Fort Union, partly due to the dilapidated state of the structure and partly due to its small size.  In 1866 more troops arrived in the area and began construction of Fort Buford, three miles east of Fort Union.

For a variety of reasons, including shifting migration patterns and decrease in numbers of buffalo, shifting tribal territories, the arrival of the military in the region, and due to changes related to westward expansion, the trade in buffalo robes began to seriously decline in the 1860's.  By 1866 Fort Union had been sold to the Northwest Fur Company (not to be confused with the North West Company), but that company could only make the trade last another year.  In 1867 the post was sold to the Army, and troops from Fort Buford dismantled Fort Union, using the material to expand Fort Buford. 

In 1987 reconstruction of the fort as a historic site began. 

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