William Mason Fassett (January 28, 1876 – March 23, 1958) was a United States Army officer in the late 19th and early 20th centuries.

After his promotion to the rank of brigadier general on October 1, 1918, Fassett assumed command of the 37th Division of the American Expeditionary Forces (AEF).

[1][2] The citation for his Army DSM reads: 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.