Westbound traffic became important to what is now Wyoming after John C. Fremont led an expedition across the area in 1842. Many mountain men found work as guides and providing supplies on newly developing trails to the West. The principal trail, which followed the Platte and Sweetwater Rivers, crossed the Continental Divide at South Pass. This was known first as the Oregon Trail. After Brigham Young led the Mormon migration through Wyoming into Utah in 1847, it became known as Mormon Trail. Later it was known as the California Trail and was used by the Pony Express and stagecoach lines until the Overland Trail opened to the south.

Ohio congressman John M. Ashley was the first to propose both the name "Wyoming" and territoriality. Wyoming was created out of what had been part of Oregon, Dakota, Utah, and Idaho territories. The building of the Union Pacific Railroad led to permanent settlements across the southern part of the state in 1867-1869. Cattle ranching grew as a business, in spite of the severe winter of 1887-1888.

Range wars between cattlemen and sheep men and increased cattle rustling led to reprisals such as the Johnson County War of 1892. Although few farmers settled in Wyoming under the Homestead Act of 1862, ranchers and cowboys filed claims to land and began fencing the better ranges and waterholes. In 1890, after nearly 22 years as a territory, Wyoming became a state.