Mountain men were trappers and explorers who roamed the North American Rocky Mountains — from about 1810 through the 1880s (with a peak population in the early 1840s) where they were instrumental in opening up the various Emigrant Trails (widened into wagon roads) allowing Americans in the east to settle the new territories of the far west by organized wagon trains traveling over roads explored and in many cases, physically improved by the mountain men and the big fur companies originally to serve the mule train based inland fur trade.

They arose in a natural geographic and economic expansion driven by the lucrative earnings available in the North American fur trade, in the wake of the various 1806–07 published accounts of the Lewis and Clark expeditions' (1803–1806) findings about the Rockies and the (ownership disputed) Oregon Country where they flourished economically for over three decades.

By the time the two new international treaties in early 1846 and early 1848 respectively officially settled new western coastal territories on the United States and spurred a large upsurge in migration, the days of many Mountain men making a good living by fur trapping had become a thing of the past. This was because the fur industry was failing due to overtrapping. Fortuitously, America's ongoing western migration by wagon train with the goal of claiming cheap lands in the west was building rapidly from a trickle of settlers from 1841's opening of the Oregon Trail (now a wagon road) to a flood of emigrants headed west by 1847–49 and thereafter well into the later 1880s.