Clark moved to Dubuque, Iowa, in 1848. Two years later, in 1850, he was elected as a Democrat to represent Iowa's 2nd congressional district, defeating Whig candidate John Parsons Cook by only 150 votes out of over 15,000 cast.[2] Clark served in the Thirty-second Congress, from March 4, 1851, to March 3, 1853. In a rematch in 1852, Cook unseated Clark. Two years later, Clark tried again to regain his seat, but was defeated.
In 1857, Clark was elected to the Iowa House of Representatives, and played an important role in adapting the laws to the new Iowa Constitution.[2] In the 1860 presidential election, he participated in the Iowa state Democratic convention (declining to follow the breakaway faction supporting the John C. Breckinridge candidacy), where he was elected as a potential presidential elector for U.S. Senator Stephen A. Douglas.[3] During the Civil War, he sided with his party's "War Democrat" faction, in opposition to the pro-secession "Mahoneyite" faction of followers of jailed newspaper editor D.A. Mahoney.
Clark eventually left Iowa to practice law in Chicago, Illinois.
He was appointed United States Register in Bankruptcy in 1866.
In 1869, he retired from active business and returned to Conway, Massachusetts.
He died in Conway on September 16, 1886. He was interred in Howland Cemetery.
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