He practiced law in Virginia and was a delegate to the Virginia Constitutional Convention of 1829-1830 and a member of the House of Delegates. He was elected to the Twenty-fifth United States Congress in 1836 as a Jackson Democrat. In 1847, he was elected to the Senate after the death of Isaac S. Pennybacker, and he was re-elected in 1850 and 1856. Mason famously read aloud the dying Senator John C. Calhoun's final speech to the Senate, on March 4, 1850, which warned of disunion and dire consequences if the North did not permanently guarantee the Southequal representation in Congress. He also complained of personal liberty laws: "Although the loss of property is felt, the loss of honor is felt still more." Mason also drafted the Fugitive Slave Law of 1850, enacted on September 18, 1850 as a part of the Compromise Measures of that year. Mason represented the majority view in leading the Senate committee that investigated the John Brown raid on Harper's Ferry in October 1859. Mason was President pro tempore of the Senate during the Thirty-fourth and Thirty-fifth Congresses but was expelled from the Senate in 1861 for support of the Confederate States. While he was traveling to his post as Confederate envoy to Britain and France on the British mail steamer RMS Trent, the ship was stopped by USS San Jacinto on November 8, 1861. Mason and John Slidell were confined in Fort Warren, Boston Harbor, precipitating the Trent Affair, which threatened to bring Britain into open war with the United States. The US public erupted with a huge display of triumphalism at this dramatic capture. Even the cool-headed Lincoln was swept along in the celebratory spirit, but when he and his cabinet studied the likely consequences of a war with Britain, their enthusiasm waned. After some careful diplomatic exchanges, they admitted that the capture had been conducted contrary to maritime law and that private citizens could not be classified as "enemy despatches." Slidell and Mason were released, and war was averted. The two diplomats set sail for England again on January 1, 1862. Mason represented the Confederacy there until April 1865. One of his first acts in London was to raise the issue of Union blockades. Until 1868, he lived in Canada, and he then returned to the Clarens Estate near Alexandria, Virginia. He died at Clarens in 1871. He was interred in the churchyard of Christ Church, Alexandria, Virginia.
Family
Marriage and children
Mason married Eliza Margaretta Chew on 25 July 1822 at Cliveden in Germantown, Pennsylvania. The couple had eight children: