The first non-Native Americans to pass through the area of Castle Lake were hunters and trappers of the Hudson's Bay Company, who began to come down the Siskiyou Trail from present-day Washington into Mexican-controlled California, in the late 1820s and early 1830s, in search of beaver and other pelt-bearing animals. At about this same time, Ewing Young led the first group of Americans up the Siskiyou Trail, passing near Castle Lake. In 1841, an overland party of the United States Exploring Expedition, the first cartographers and scientists came through the area. Following the annexation of California by the United States in 1848 as a result of the Mexican–American War, Castle Lake came under the control of the U.S. Federal Government as public land.