Cassius Mareellus Clay, an American politician, son of Gen. Green Clay, born in Madison co., Kentucky, Oct. 19, 1810. He graduated at Yale college in 1832, practised law in Kentucky, and was elected to the state legislature in 1835 and 1837. Removing to Lexington, he was again elected to the legislature in 1840, but was defeated in 1841 on account of his opposition to slavery. In 1844 he made a tour of the north to advocate the election of his relative Henry Clay to the presidency. In 1845 he issued in Lexington the first number of the "True American," a weekly anti-slavery newspaper; but the press was seized by a mob and sent to Cincinnati. The journal was afterward revived by Mr. Clay, who printed it in Cincinnati and published it in Lexington. Upon the breaking out of the war with Mexico in 1846, he entered the service as captain, and was taken prisoner at Encarnacion, Jan. 23, 1847. In 1848 he supported the nomination of Gen. Taylor for the presidency. The question of changing the state constitution was discussed the next year, and by the influence of Mr. Clay a large convention of emancipationists assembled at Frankfort. In 1850 he separated from the whig party and became the anti-slavery candidate for governor, receiving about 5,000 votes.

On the accession of President Lincoln in 1861 he was appointed minister to Russia. Returning to the United States, he was commissioned major general of volunteers April 11, 1862, but resigned March 11, 1863, and was reappointed minister to Russia, which position he held till 1869. In 1870 he publicly supported the cause of the Cuban revolutionists, and in 1872 the election of Horace Greeley as president. A volume of his speeches, edited by Horace Greeley, was published in 1848.