While traveling to Europe 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 that threatened to bring Britain into open war with the United States.
After some careful diplomatic exchanges, they USA 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 for until April 1865. One of his first acts in London was to raise the issue of Union blockades. Ultimately, no nation ever granted recognition to the Confederacy.
Mason lived in exile in Canada from 1865 until 1868, when he returned to the USA.