Fassett spent two years as a student officer at Fort Leavenworth, graduating from the Army School of the Line in 1908 and the Army Staff College in 1909.[4][5]
The President of the United States of America, authorized by Act of Congress, July 9, 1918, takes pleasure in presenting the Army Distinguished Service Medal to Brigadier General William Mason Fassett, United States Army, for exceptionally meritorious and distinguished services to the Government of the United States, in a duty of great responsibility during World War I. In the forcing of the crossing of the Escaut River, Belgium, in November 1918, and the establishment of a bridgehead thereat, General Fassett demonstrated his ability as a leader. The successful operation of his brigade in this and in ensuing actions were greatly influenced by his efforts.
After reverting to his permanent rank of colonel, Fassett retired from the military in January 1924. Congress restored his brigadier general rank in June 1930.[5] As a retiree, Fassett raised citrus fruit in Florida, and he died in Orlando on March 23, 1958.[1][2]