In 1838, after his commission as second lieutenant in the U.S. Army Corps of Engineers, John Charles Fremont worked as a surveyor in the mountains of Carolina, and joined French explorer Joseph Nicolas Nicollet on an expedition that mapped the region between the upper Mississippi and Missouri rivers. In 1841, he commanded a U. S. Topographical Corps (later the U. S. Army Corps of Engineers) survey of the Des Moines River in Iowa. Between 1842 and 1845 Fremont led three surveys of Oregon Territory where he met frontiersman Kit Carson, who became the guide for his expeditions.
During his first western expedition in 1842, he explored the Kansas and Platte rivers, charted the location of South Pass, Wyoming, and climbed Fremont Peak in the Wind River Mountains. In 1843, on his second western expedition, he explored the Medicine Bow Mountains of southern Wyoming, and was unsuccessful in his search for an alternate east-west route through the Rocky Mountains. He continued on to explore the Great Salt Lake, and the eastern Cascades to Pyramid Lake, Nevada. This expedition lasted into 1844, exploring Utah’s Great Basin, the Wasatch and Uinta mountains, as well as Utah Lake, and locating Muddy Pass through the middle Rockies and charting the Great Basin.
Fremont's third western expedition, in 1845-47, crossed the Great Basin to Walker Lake, and the Sierra Nevada into Northern California. After exploring Klamath Lake, he returned to California to take part in the Mexican-American War of 1846-48. Following the war, he undertook his fourth western expedition, seeking a route to the upper Rio Grande River through Sangre de Cristo and the San Juan Mountains of northern New Mexico.
In 1850, Fremont was elected as one of the first two senators from the state of California, but by 1853 he was on his fifth expedition, returning to the Wasatch Mountains and the Sierra Nevada scouting for a proposed railroad through northern Utah.
Fremont was the first presidential candidate chosen by the newly formed Republican Party during the election of 1856, but he was defeated by Democrat James Buchanan. In 1864, he was again a presidential nominee, but he withdrew in favor of incumbent President Abraham Lincoln. He also served as the territorial governor of Arizona from 1878 to 1883.
Fremont's expeditions and maps made possible the great migration of settlers to the American west. The Oregon Trail would have been an unknown passage if it weren't for his surveying and mapping. Indeed, western settlement would have proceeded far differently if it weren't for Fremont and his intrepid explorations.