Custer County was created by the Colorado legislature on March 9, 1877, out of the southern half of Fremont County. Originally set in Ula, the county seat moved to Rosita in 1878, and to Silver Cliff in 1886 before settling in Westcliffe in 1928. It was named in honor of Lt. Colonel George Armstrong Custer, who had died the previous year.
The county was the site of a silver rush during the 1870s. Thousands of men poured into the county during this time in the hunt for silver. Some of the notable mines include the Geyser Mine (on the north edge of the town of Silver Cliff), the Bassick Mine (near the ghost town of Querida) and the Bull Domingo (north of Silver Cliff).
During the late 19th century a railroad line was connected through the Grape Creek Canyon but was permanently closed after a few disastrous floods. The old railhouse has been turned into a historical landmark in the town of Westcliffe.
After the mines were exhausted, the population dropped considerably and was replaced by cattle ranchers. An extensive system of irrigation ditches was built throughout the valley. Ranching in the Wet Mountain Valley continues to this day.