William was born on the 6th of April 1859, in Centerville, New York, and he attended the US Military Academy at the age of 19. He graduated in 1882, commissioning as a Second Lieutenant with the 5th Infantry Regiment in Washington state and was also a professor of military science for a year. He married Elizabeth in 1885 and the couple had two children. In both the Spanish-American and Philippine-American Wars, William was an aide-de-camp to Colonel Samuel Overshine but his actions as a Captain during the Battle of Zapote River later earned him the Medal of Honor. The citation reads:

With nine men volunteered to hold an advanced position and held it against a terrific fire of the enemy estimated at 1,000 strong. Taking a rifle from a wounded man, and cartridges from the belts of others, Capt. Sage himself killed five of the enemy.

William was awarded the Medal of Honor on the 24th of July 1902, and he went on to attend the Army War College, where he graduated in 1907. He performed various assignments in the Philippines, Alaska, and New York, before deploying to Texas during the Pancho Villa Expedition as a Lieutenant Colonel. When World War 1 began, William was a Brigadier General and was placed in command of the Officer Candidate School in Fort Snelling, Minnesota, before commanding the 38th Division at Camp Shelby, Mississippi. He deployed to Europe in command of the 2nd Brigade, American Expeditionary Forces, for post-war occupation of France and Germany. His final duty station was as a Major General in command at Fort Russell in Wyoming, when on the 3rd of June 1922, William Hampden Sage died at the age of 63 while traveling to Walter Reed Hospital. He was one month away from retirement and he is buried in Arlington National Cemetery: Section 2, Lot 913.