Thornton was a member of a pioneering Ohio and Indiana family that had migrated to the region from South Carolina over opposition to slavery.
According to Thornton's sister, Minerva Thornton Bickell, James J. Thornton was an avid reader and left home at the age of 19 to pursue an education: he worked constantly and studied, spending a term at Bloomington College (now, University of Indiana at Bloomington) and teaching preparatory classes (1835) [REF].