He graduated from Washington College in Washington, Pennsylvania and was admitted to the bar in 1843, commencing practice in Wheeling.
He was a cousin to author Samuel L. Clemens (aka Mark Twain). He has a town named after himself which is in Marshall County West Virginia. Born in Wheeling, Virginia (now West Virginia), Clemens was appointed a cadet to the United States Military Academy in West Point, New York, but resigned after six months.
He was elected a Democrat to the United States House of Representatives to fill a vacancy in 1852, serving until 1853.
Clemens was later elected back to the House in 1856, serving again from 1857 to 1861. Clemens later moved to Saint Louis, Missouri and resumed practicing law until his death there on June 30, 1881.
He was interred in Calvary Cemetery in Saint Louis. Clemens fought a duel with O. Jennings Wise, the son of Virginia Governor Henry A. Wise.
Wise was uninjured in the duel, but Clemens received a severe injury to his right testicle.
He was a member of the Virginia Convention in 1861 and afterwards resumed practicing law in Wheeling.
He was not favorably impressed by Abraham Lincoln, whom he called "a cross between a sandhill crane and an Andalusian jackass." "He is vain, weak, puerile, hypocritical, without manners, without moral grace, and as he talks with you he punches you under your ribs." Clemens also wrote, "He is surrounded by a set of toad eaters and bottle holders." During the Civil War, he opposed secession.