He was born in Lynchburg, Virginia to Pleasant Thurman and Mary Granberry Allen Thurman. Both his parents were teachers, his father also a Methodist minister. In 1815 his parents emancipated their slaves and moved to Chillicothe, Ohio. He attended the academy run by his mother, and then studied law as an apprentice to his uncle, William Allen (who later became a Senator from Ohio).
In 1835 he was admitted to the Ohio bar and became his uncle's law partner. In 1837 his uncle entered the Senate. On November 14, 1844 Thurman married Mary Dun Thomplins (or Tompkins), who was to bear him three children. The same year he was elected to the House of Representatives as its youngest member. He generally supported the majority of the Democrats on all issues except internal improvements, on which he tended to vote with the Whigs. He supported the Polk Administration's conduct of the Mexican War, spoke in favor of the 54o40' northern limit to the Oregon territory, and voted for the Wilmot Proviso, which would have banned slavery from the territory gained from Mexico. Ironically, his support for the latter was due to anti-African-American prejudice, as he wanted to reserve this territory for white settlement.
After a single two-year term, he left the House voluntarily to resume private law practice. However, in 1851, he accepted an appointment to the Ohio supreme court, where he served for five years, the last year as the chief justice. He then returned to private law practice.
Thurman spoke out against the repeal of the Missouri Compromise and opposed the pro-slavery Lecompton constitution for Kansas. In 1860 he was a supporter of Stephen A. Douglas for President. He never accepted the right of a state to secede, but he felt it was unwise to fight a state that had already left the Union, so during the American Civil War, he was opposed to Lincoln's policies, especially on emancipation. While he supported the war effort, he encouraged compromise and a political settlement. In 1867 he ran for Governor of Ohio, on a platform opposed to extending suffrage to blacks, but lost to Rutherford B. Hayes in a close election. The Ohio voters chose a Democratic state legislature, however, which selected Thurman as Senator for the term beginning in 1869. He there became a strong opponent of the Republicans' Reconstruction measures.
In 1873 Thurman crafted a strategy that led to Ohio choosing once more a Democratic legislature, and electing Thurman's uncle William Allen as governor. The legislature elected Thurman to another term in the Senate. During the twelve years he served in the Senate, he became the leader of the Democrats in that body.
In the 1876-1877 electoral college crisis, he helped to arrive at the solution of creating the Electoral Commission to settle the controversy, and ultimately served as one of the members of the commission, as one of the five Senators (one of the two Senate Democrats, and one of the seven Democrats altogether). As a Democrat, he voted with the seven-member minority, in favor of the Samuel J. Tilden electors in all cases, but the Republican majority prevailed in all the votes, and Thurman's 1867 gubernatorial opponent, Rutherford B. Hayes, became President. (One of the House of Representatives' members of the Commission, fellow Ohioan James Garfield, was to become the President four years later, after being chosen by the now-Republican Ohio legislature to succeed Thurman.)
In the Senate, Thurman served on the Judiciary Committee, becoming its chairman when the Democrats won control of the Senate in the 46th Congress. He also became president pro tempore of the Senate briefly, serving as president of the Senate because of the illness of Vice-President William A. Wheeler, before Ohio chose a Republican legislature, which would not reelect Thurman. They first chose Garfield, but on his election to the Presidency, selected John Sherman to succeed Thurman beginning in 1881. Garfield did appoint Thurman as American representative to the international monetary conference in Paris.
Thurman was put forth as a favorite son candidate in the Democratic presidential nominating conventions in 1876, 1880, and 1884. In 1888, he was selected by Grover Cleveland as his Vice Presidential running mate, but the ticket lost to the Republicans.
Thurman died at home in Columbus.