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.

At the age of eighteen, Thurman worked on a land survey, and at twenty-one became private secretary to the Governor of Ohio, Robert Lucas.

In 1867, he ran for Governor of Ohio, on a platform opposed to extending suffrage to blacks, but lost to future U.S. president Rutherford B. Hayes in a close election.

"[9] During the campaign, Thurman appealed to white supremacist attitudes in virulent tones, vowing to fight "the thralldom of n*****ism.

"[10] In Ohio, like most Northern states, black suffrage referendums failed to pass due to small fractions of Republican voters joining Democratic opposition.

The Ohio voters chose a Democratic state legislature, however, which selected Thurman as senator for the term beginning in 1869.

He was known for constant hard work, good preparation, and courteous treatment of his opponents, and other members ranked him among the top three senators of his time, in terms of ability.

He has fine passing power of cutting up his political opponents, saying a word of encouragement to some Republican when he is down, and scattering the caucuses of the opposite side with a pistol shot.

"He would wave his red bandana pocket handkerchief like a guidon, give his nose a trumpet-blast, take a fresh pinch of snuff, and dash into the debate, dealing rough blows, and scattering the carefully prepared arguments of his adversaries like chaff," a Washington long-time reporter remembered.

Journalists told how at a given signal—a long blow of his nose—he would get ready to exit the Senate so that the two could meet in the Judiciary Committee room to share a liberal amount of Kentucky Bourbon.

[16] An advocate of free trade,[17] Thurman bemoaned of protective tariffs as a taxation upon "everything one wears from the crown of his head to the soles.

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.

Garfield did appoint Thurman as American representative to the international monetary conference in Paris, a selection that Republican senators welcomed: they regretted his departure from among them.

[20] Thurman spent his retirement reading French novels in the original language, playing whist, and amusing himself with mathematical problems; he had a reputation as one of the best mathematicians in Ohio.

Thurman's appeal came from his popularity among old-line Democrats, distrustful of Grover Cleveland's liberalism, and his known hostility to railroad monopolists.

[21] In the concurrent 1884 United States Senate election in Ohio, Thurman did not oppose the campaign of incumbent Democratic senator George H. Pendleton, who faced opposition from the statewide party due to his sponsoring of the Pendleton Civil Service Reform Act that ended patronage politics in the United States.

[27][25] Following the 2021 United States Capitol attack, Democrats charged that Donald Trump was responsible for the violence and sought to impeach and remove him from office.