Tanner would rise through ranks to become the unrivaled boss of a powerful political machine. In 1870 he began his long and successful political career in the Illinois Republican party of that era, winning elections and garnering appointments to increasingly important positions. He was first elected to the office of sheriff of Clay County, Illinois (1870–72), and afterwards became circuit court clerk for the county (1872–76). He was appointed master in chancery (i.e., a senior officer of the court) of the circuit court (1877–80), and then served two terms in the Illinois State Senate (1880–1883). Tanner was appointed United States marshal of the Southern District of Illinois by Republican President Chester A. Arthur, but was soon removed by incoming President Grover Cleveland.