In 1815, he began study of law in the offices of Joshua Collett,[4] He was admitted to the bar in 1817, commencing practice in Lebanon; he was prosecuting attorney of Warren County from 1818 to 1828.

[8] 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 the office of Governor of Ohio.

Known for his sharp wit, debating skills, and endless campaigning, he was elected Governor in 1840, defeating incumbent Wilson Shannon.

[9] Corwin was also a member of the United States Senate, having been elected by the Ohio General Assembly as a Whig and served from March 4, 1845, to July 20, 1850.

[10] Thomas Corwin, as quoted by Canadian humorist Stephen Leacock: The world has contempt for the man who amuses it.

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."

In 1857, former Ohio Governor William Bebb shot a man and was tried in 1858 for manslaughter in Winnebago County, Illinois, where he lived.

In 1860, he was chairman of the House "Committee of Thirty-three", consisting of one member from each state, and appointed to consider the condition of the nation and, if possible, to devise some scheme for reconciling the North and the South in the secessionist crisis following the election of Abraham Lincoln to the presidency.

[12]Corwin's amendment restated what most Americans already believed, that under the Constitution the Congress had no power to interfere with slavery in the states where it existed.

Corwin was reelected to the House of Representatives in 1860 but resigned on March 12, 1861, after being appointed by the newly inaugurated President Lincoln to become Minister to Mexico, where he served until 1864.

[14] After resigning from his post as Minister, Corwin settled in Washington, D.C. in 1864, and practiced law until his death on December 18, 1865, at age 71.

According to Benjamin Perley Poore, Corwin was "a born humorist": He [Corwin] was a middle-sized, somewhat stout man, with pleasing manners, a fine head, sparkling hazel eyes, and a complexion so dark that on several occasions – as he used to narrate with great glee – he was supposed to be of African descent.

"There is no need of my working," said he, "for whenever I cannot support myself in Ohio, all I should have to do would be to cross the river, give myself up to a Kentucky negro-trader, be taken South, and sold for a field hand.