A mountain man is a male trapper and explorer who lives in the wilderness. Mountain men were most common in the North American Rocky Mountains from about 1810 through the 1880s (with a peak population in the early 1840s). 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 two new international treaties in early 1846 and early 1848 officially settled new western coastal territories on the United States and spurred a large upsurge in migration, the days of mountain men making a good living by fur trapping had largely ended. This was partially because the fur industry was failing due to reduced demand and over trapping. With the silk trade and quick collapse of the North American beaver-based fur trade in the later 1830s–1840s, many of the mountain men settled into jobs as Army Scouts, wagon train guides and settlers through the lands which they had helped open up. Others, like William Sublette, opened up fort-trading posts along the Oregon Trail to service the remnant fur trade and the settlers heading west.