Mason was born at Okeley Manor in Fairfax County, Virginia on September 1, 1834.^[1][2] His parents were Dr. Richard Chichester Mason and Lucy Bolling Randolph.^[1][2] He was a great-grandson of George Mason, author of the Virginia Bill of Rights.^[2][3]

After graduating from school, Mason went to Philadelphia, Pennsylvania where he pursued business.^[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 became quartermaster in 1864.^[5]

After the war, Mason engaged in business and, then, took up the profession of teaching.^[2] He taught at the United States Military Academy in West Point, New York.^[2] He returned Virginia and taught mathematics and Latin at the Norwood Institute.^[6] In 1892, he founded Gunston Hall School for Young Ladies at his home in Wasington, D.C.; the school was named for the homestead of his great-grandfather George Mason.^[2][3] Mason and his wife opened their school ifor their children and the children of their intimate friends.^[6][3]

Gunston Hall School moved to a larger campus and operated as a boarding school for young women for fifty years.^[3] After its closure, the building housed Epiphany School, an Episcopal institution.^[3] The building is currently the home of the National Museum of American Jewish Military History. Mason's character impressed itself upon his students and his influence among them was widely felt and acknowledged by the students of successive years.^[2]

Mason married Elizabeth "Bettie" Harrison Nelson at St. Stephen's Church on August 18, 1875.^[1][2][5] She was the daughter of daughter of Keating Lewis Simmons Nelson and his wife Julia Ann Rogers of Albemarle County, Virginia.^[1][2] The couple had six children:^[1][5]

• Richard Nelson Mason (26 June 1876–22 November 1940)
• Julia Nelson Mason Matthews (23 January 1878–27 December 1964)
• Lucy Randolph Mason Moffett (31 January 1880–1 April 1965)
• Margaret Thornton Mason (7 February 1882–February 1884)
• Mary Wallace Mason Patchin (26 April 1884–28 August 1963)
• Susan Josephine Beverley Mason Easley (17 January 1888–31 July 1962)

In 1879, the famly moved to Washington, D.C. to a large mansion at 3017 O Street, N.W., previously the home Commodore Stephen Cassin.^[6][3][4]

Mason died on April 22, 1910 in Washington, D.C. at age 75.^[1][2] His funeral, which took place on April 24, at St. Margaret's Episcopal Church in Washington, D.C.^[2] The Reverend Herbert Scott Smith and the Reverend Samuel A. Wallis conducted the services.^[2]Mason was buried at Ivy Hill Cemetery in Alexandria, Virginia.^[1][2] The Robert E. Lee Camp of Confederate Veterans of Alexandria attended his burial in uniform.^[2]