John C. Fremont was born in Savannah, Georgia, in 1813, the son of Charles Fremon, a French emigre, and Ann Beverly Whiting of Virginia. Frémont spent his boyhood in Charleston and was educated in the Scientific Department of the College of Charleston before his expulsion in 1831, three months short of graduation.

In 1833 Frémont obtained a civilian post as teacher of mathematics to midshipmen; 1836-37 found him assisting in the surveys of the projected Charleston and Cincinnati Railroad and in the Cherokee country; and in 1838 he was commissioned a second lieutenant in the United States Corps of Topographical Engineers and assigned to accompany the French scientist Joseph N. Nicollet on a two-year reconnaissance of the Minnesota country. Under Nicollet's tutelage, Frémont quickly absorbed a great deal of information about science and sophisticated methods of geodetic surveying as well as about how to organize and manage an expedition. When the two returned to Washington to work on the report and map of the survey, he met Jessie, the talented daughter of Senator Thomas Hart Benton, with whom he eloped in October 1841. The alliance was to prove extremely valuable to the advancement of his career in exploration and politics.