After the American Civil War, Tillman was elected Governor of South Carolina from 1890 to 1894, and established Clemson College and Winthrop College while in office.
He became a member of the State constitutional convention in 1895.
He was elected as a United States Democratic Party member to the United States Senate in 1894, and was reelected in 1901, 1907, and 1913, thus serving from the day he took office, 4 March 1895, until his death. An outspoken populist, Tillman was known as "Pitchfork Ben" during his years in the Senate. During that time period, he was censured by the Senate in 1902 after assaulting another Senator on the Senate floor, became the chairman of the Committee on Revolutionary Claims (57th through 59th Congresses), served on the Committee on Five Civilized Tribes of Indians (61st and 62nd Congresses), and the Committee on Naval Affairs (63rd through 65th Congresses). During and after World War I, impatient with the Navy's requests for larger battleships every year, he ordered the United States Navy to design "maximum battleships," the largest battleships that they could use.
Tillman died in Washington, DC, on 3 July 1918 and is buried in Ebenezer Cemetery, Trenton, South Carolina.