Corwin, whose brother Moses Bledso Corwin and nephew Franklin Corwin were also U.S. Representatives, was born in Bourbon County, Kentucky and moved with his parents to Lebanon, Ohio in 1798. He studied law, and was admitted to the bar in 1817, commencing practice in Lebanon; he was prosecuting attorney of Warren County from 1818 to 1828.

In 1822-1823 and 1829, he was a member of the Ohio House of Representatives; in 1830 he was elected as a Whig to the U.S. House of Representatives and served from March 4, 1831, until his resignation, effective May 30, 1840, having become a candidate for Governor of Ohio. Known for his sharp wit, debating skills and endless campaigning, he was elected Governor in 1840, defeating incumbent Wilson Shannon. Shannon defeated Corwin in a rematch just two years later. Corwin was also a member of the United States Senate, having been appointed by the Ohio General Assembly as a Whig and served from March 4, 1845 to July 20, 1850.

He resigned from the Senate to become President Fillmore's Secretary of the Treasury shortly after the death of President Taylor. Like his immediate predecessor, William M. Meredith, Corwin believed in a protective tariff, but he did not want to make sudden or drastic changes in the free-trade tariff law of 1846. He objected to that law's provisions, which taxed some imported raw materials at a higher rate than the imported manufactured goods made from those materials, stating in a report to Congress that such provisions certainly take from the manufacturer and artisan that encouragement which the present law was intended to afford. As a longtime Whig, however, Corwin was unsuccessful in passing any tariff legislation in a Congress controlled by Democrats. He retired as Secretary at the end of Filmore's administration.

He was again elected to the House of Representatives in 1858, and returned to that body as a Republican and served from March 4, 1859 to March 12, 1861. He resigned only a few days into the 37th Congress after being appointed by the newly inaugurated President Lincoln to become Minister to Mexico, where he served until 1864. Corwin, well-regarded among the Mexican public for his opposition to the Mexican War while in the Senate, helped keep relations with the Mexicans friendly throughout the course of the Civil War, despite Confederate efforts to sway their allegiances.

After resigning from his post as Minister, he settled in Washington, D.C., and practiced law until his death in December 1865. His interment was in Lebanon Cemetery, Lebanon, Ohio.

Thomas Corwin is perhaps best known for his successful sponsorship during the 36th Congress in early 1861 of the proposed Corwin amendment to the United States Constitution which remains to this day technically still pending for ratification before the state legislatures. That amendment would have prohibited any amendments to the Constitution from interfering with slavery in the United States. When it was approved by Congress and sent out to the state legislatures for consideration, it was a last-ditch effort to avert the outbreak of the Civil War.