The most important fur trading post on the upper Missouri River, Fort Union controlled the bison robe and fur trade over a huge area encompassing what are now North and South Dakota, Montana, and Wyoming from 1828 to 1867. Here, Assiniboine, Crow, Cree, Ojibway, Blackfeet, Hidatsa, and other tribes traded buffalo robes and other furs for trade goods such as beads, guns, blankets, knives, cookware, and cloth.

During its heydays the trading post employed as many as 200 people, mostly Métis Indians, of French and Native American ancestry.

The rivers were the main source of travel for the fur traders, primarily utilizing keelboats until 1832 when the first steamboat, the Yellowstone landed here. Mackinaw boats were another form of travel made by the workers of the fort and bull boats, constructed by the tribes out of a green buffalo hide and willow, were also used.

In 1867, Fort Union was sold to the U.S. Army and the post was razed. Its building materials were used to construct the Fort Buford about two miles away. The site of the longest running American fur trading post in history then lay abandoned and largely neglected for a century.

However, in 1866, local interest in the historical site led Congress to establish the Fort Union Trading Post National Historic Site as a part of the National Park System. After three archaeological projects, the trading post was partially reconstructed from 1986 to 1989 with structures appearing as they did in 1850-1851.

Today, the reconstructed Fort Union represents a unique era in American history, a brief period when two different civilizations found common ground and mutual benefit through commercial exchange and cultural acceptance.