He was elected as a Republican to the United States Senate to fill the vacancy caused by the resignation of William Branch Giles, despite being constitutionally underage for the office.

He then moved to Loudoun County, Virginia where he was an unsuccessful candidate for election to the Fifteenth Congress (1817).

It was a bitter campaign that gave rise to several duels: Mason himself was later killed in a duel with his second cousin, John Mason McCarty, at Bladensburg Duelling Field, Maryland, as a result of this campaign.

He is buried in the churchyard of the Episcopal Church at Leesburg, Virginia.

[1][2] Mason married on 1 May 1817 to Charlotte Eliza Taylor (died 1846) at Dr. Charles Cocke's in Albemarle County, Virginia.