Part II. John Charles Fremont (1813-1890) In 1842, together with Kit Carson, he mapped the Oregon Trail. Subsequent Western exploratory expeditions - in 1843-44 and 1845-46 - took him beyond the Rocky Mountains and the Sierra Nevada, into California. Fremont was the U.S. Senator for California, in 1850-51; in 1856, he was the Republican nominee for President of the United States.