Lake County was one of the original 17 counties created by the Colorado legislature on November 1, 1861. As originally defined, Lake County included a large portion of western Colorado to the south and west of its present boundaries. The county was named for Twin Lakes.
Placer gold was found at Colorado Gulch in 1863 as part of the Colorado Gold Rush.
Lake County slowly lost territory over the succeeding decades, losing land in its southeast to Saguache County in 1866 and Hinsdale Countyin 1874; in its southwest to La Plata County in 1874 and San Juan County in 1876, and in its west to Ouray and Gunnison counties in 1877.
With its many reductions in size, Lake County's designated county seat also changed multiple times within just a few years, residing successively in Oro City (from 1861), Lourette (from 1863), Dayton (from 1866), and Granite (from 1868).
By 1878, Lake County had been reduced to an area including only present-day Lake and Chaffee counties. On February 8, 1879, the Colorado legislature renamed Lake County as Carbonate County, although this designation name only lasted for two days, until Chaffee County was split off from Carbonate's southern section on February 10 and the remaining northern portion was redesignated Lake County with its current county seat of Leadville.