He was the ninth Governor of Rhode Island from 1817 to 1821 and president of the Roger Williams Bank from 1817 to 1854.

Knight was elected in 1821 as a Democratic-Republican to the U.S. Senate to fill the vacancy caused by the death of James Burrill, Jr.; he was re-elected in 1823 as a Crawford Republican, in 1829 as an Anti-Jacksonian, and again in 1835 as a Whig, and served from January 9, 1821, to March 3, 1841.

While in the Senate he was chairman of the Committee to Audit and Control the Contingent Expenses (Twenty-second, Twenty-third and Twenty-sixth Congresses) and a member of the Committee on Manufactures (Twenty-fourth Congress).

Knight retired from public life and was a delegate to the State constitutional convention in 1843.

He died in Providence in 1854; interment was in Grace Church Cemetery.