[4] At the onset of the American Civil War, Mason volunteered as a private in the Black Horse Cavalry of Fauquier County, Virginia.

[2][5] Soon afterward, Mason was detailed to act as commissary sergeant and supplied food to his command.

[2] He gained a captain's commission in the 4th Virginia Cavalry, and later a major's rank as assistant commissary in Fitzhugh Lee's division.

[2] Mason married Elizabeth "Bettie" Harrison Nelson at St. Stephen's Church on August 18, 1875.

[1][2] The couple had six children:[1][5] In 1879, the family moved to Washington, D.C. to a large mansion at 3017 O Street, N.W., previously the home Commodore Stephen Cassin.