Robert Lee Thornton, Sr. (often R. L. Thornton; 10 August 1880 – 15 February 1964) was an American banker, civic leader, and four-term Mayor of Dallas, Texas. Thornton had little schooling; his early years were divided between school and farm labor. Later, he was a store clerk and a traveling salesman. After two unsuccessful business ventures, Thornton began a banking operation in Dallas in 1916, financed by loans from family. The bank progressed to be a Texas-wide institution, and by 1923 it had a national charter. Thornton served as president (1916–1947) of the bank he founded, the Mercantile Bank and Trust Co, and board chairman (1947–1964).