He was sent to Ohio University at Athens, 1820-22, by his elder brothers. Later he studied at Transylvania University.
At Lexington he read law in the office of his brothers. He returned to Ohio in 1826, was admitted to the bar in 1830, and began to practise at St. Clairsville. In 1832 he was defeated for Congress on the Democratic ticket, but the next year he was elected state's attorney.
He was chosen governor of Ohio in 1838, being the first native to attain that office. He was defeated for reelection two years later by the popular Thomas Corwin but in the contest of 1842 he defeated Corwin.
Shannon resigned the governorship in 1844 to become minister to Mexico (appointed April 9). In that position he "blustered, blundered, threatened and undertook to argue" (J. H. Smith, The War with Mexico), and although Calhoun disapproved his tactless course he was not recalled until the last of March 1845.
Upon his return Shannon practised law at Cincinnati, but abandoned his profession to lead an expedition of "Forty-Niners" from eastern Ohio and western Virginia to California. Returning to Ohio in two years, he served without distinction in Congress, 1853-55, and voted for the Kansas-Nebraska bill.
Shannon was commissioned governor of Kansas Territory on August 10, 1855. He was welcomed at Westport and Shawnee by members of the Missouri party, with whom he became confidential. Shannon himself presided at a pro-slavery meeting at Leavenworth November 14 which organized a "Law and Order" party. A crisis arose two weeks later with the outbreak of the Wakarusa War. Free-State men rescued one of their number whom Sheriff Samuel J. Jones of Douglas County had arrested, and that officer requested 3, 000 men of the Governor to enforce the laws. The militia which Shannon ordered to report for service was only partially organized, but some 1, 200 Missourians responded and assembled on the Wakarusa River, eager to destroy Lawrence. Free-State emissaries soon convinced Shannon that such was the purpose of the "border ruffians, " and he sought aid of Col. E. V. Sumner, commander of federal forces at Fort Leaven-worth.
Sumner refused to move without orders from Washington, and Shannon went to Lawrence to prevent a collision. He signed a "treaty" with Charles Robinson and James H. Lane, 1814-66, in which the two Free-State leaders pleaded ignorance of any organization to resist the laws, and Shannon denied that he had called the Missourians to assist him.
When guerrilla bands again assembled before Lawrence in May 1856, Shannon refused to intervene, and they pillaged the town and destroyed the Free-State hotel and printing presses. On June 4 he issued a proclamation commanding that armed combinations organized to resist the laws disband.
Later in June he left Kansas for an official visit to St. Louis, but directed Colonel Sumner to disperse the "pretended" Topeka legislature, by force if necessary, should it reassemble on July 4. In August, Lane invaded the territory with his "Army of the North" and attacked pro-slavery strongholds. Shannon again played the role of peacemaker and effected a settlement which constituted his last official act.
On August 18 he forwarded his resignation to the President; three days later he received notice of his removal. He resumed the practice of law, first at Lecompton, later at Topeka, and finally at Lawrence, and became a leading member of the Kansas bar.
He died in 1877.
Shannon was known for his Southern sympathies.
Quotes from others about the person
He was described by a contemporary as "an extreme Southern man in politics, of the border ruffian type. "
He was twice married; his first wife, Elizabeth Ellis, lived only a short time after their marriage; his second, Sarah Osbun of Cadiz, survived him four years.