Born in Lynchburg, Virginia, he and his family moved to Chillicothe, Ohio, when Thurman was young. Thurman established a legal practice in Chillicothe with his uncle, William Allen, who later represented Ohio in the U.S. Senate. Thurman won election to the House of Representatives in 1844, becoming the youngest member of that body. He supported the James K. Polk administration during the Mexican–American War and voted for the Wilmot Proviso, which would have banned slavery from any territory gained from Mexico. He served a single term in the House before joining the Supreme Court of Ohio. He won election to the Senate in 1869, becoming an opponent to the Republican Reconstruction policy. During the disputed 1876 presidential election, Thurman helped establish the Electoral Commission. Thurman lost re-election in 1881 as the Republicans had won control of the Ohio legislature.