Released after two months, Mason continued his voyage, and assisted Confederate purchases from Britain and Europe but failed to achieve their diplomatic involvement.

[8] Mason soon began his political career, well before his father's death, winning election several times as one of Frederick County's (part-time) representatives in the Virginia House of Delegates.

After veteran legislator Hierome L. Opie, one of the four joint delegates of Frederick and neighboring Jefferson County to the Virginia Constitutional Convention of 1829-1830, resigned, Mason took his place alongside John R. Cooke, congressman Alfred H. Powell and fellow delegate Thomas Griggs Jr.

Although some had hoped that convention would limit slaveholder power, the resulting constitution only gave additional votes to western Virginians (including those in Frederick County and those counties which would secede and become West Virginia during the American Civil War), so Mason and Castleman were re-elected and joined by William Wood for the 1830-1831 legislative session.

Voters in Virginia's 15th congressional district elected Mason as his successor in the Twenty-fifth United States Congress.

"[13][14]: 260 He so believed in the beneficence of slavery that, unlike many others in Frederick County, Mason refused to support the colonization project that led to the founding of Liberia.

Mason wrote the Fugitive Slave Law of 1850, arguably the most hated and openly-evaded Federal legislation in U.S. history.

[12]: 101 Mason strongly favored the South's "immediate, absolute, and eternal separation" if anti-slavery, Republican candidate John C. Frémont were elected president in 1856.

[12]: 83 In 1861 Mason worked behind the scenes to enable Virginia's secession, remaining in the Senate because he could get information useful for the seceding states, a type of spy behind enemy lines.

While Mason sailed toward England as a Confederate envoy to Britain on the British mail steamer RMS Trent, the ship was stopped by USS San Jacinto on November 8, 1861.

Mason and fellow Confederate diplomat John Slidell were confined in Fort Warren in Boston Harbor.

The Trent Affair threatened to bring Britain into open war with the United States, despite triumphant rhetoric in the north.

Even the cool-headed Lincoln was swept along in the celebratory spirit, but enthusiasm waned when he and his cabinet studied the likely consequences of a war with Britain.

After careful diplomatic exchanges, they admitted that the capture was contrary to maritime law and that private citizens could not be classified as "enemy despatches".

After Britain issued its refusal in 1863, he moved to Paris, continuing his unsuccessful search for a nation that would recognize or assist the Confederacy.

Learning of Mason's pro-slavery activism and his authorship of the hated Fugitive Slave Act, the soldiers, on their own initiative, set about destroying Selma.

After sanctions on Confederate officials were lifted, he returned to the United States, and bought the Clarens Estate, on 26 acres (11 ha), today in Alexandria, Virginia.

Still another type was represented to me by Senator Mason of Virginia, a thick-set, heavily built man with a decided expression of dullness in his face.

I mean the senator from Virginia [Mr. Mason], who, as the author of the Fugitive Slave Bill, has associated himself with a special act of inhumanity and tyranny.