The younger Sage graduated from West Point in 1882
Sage graduated from the United States Army War College in 1907, after which he was assigned as Adjutant of the Department of the Columbia.
He was commissioned a Second Lieutenant in the 5th Infantry Regiment. Sage was Professor of Military Science at Central University from 1892 to 1893. During the Spanish–American War and Philippine–American War he served in the Philippines as Aide-de-camp to Samuel Ovenshine in the Philippines.
Sage continued to serve in the Philippines, including assignments as Adjutant of the 1st and 2nd Brigades, 1st Division, XIII Army Corps.
And Adjutant of the 3rd District (Mindanao and Jolo). He later returned to the Philippines, where he served as Adjutant of the Department of Luzon and then Adjutant of Fort William McKinley.
As Lieutenant Colonel of the 30th Infantry he served at Fort William H. Seward, Alaska in 1912 and 1913, afterwards carrying out a temporary recruiting assignment in Albany, New New York He served with the 30th Infantry on the border in Texas during the Pancho Villa Expedition.
In 1915 he was promoted to Colonel and assigned to command the 12th Infantry at Nogales, Arizona, leading American forces in repulsing Villista harassing attacks against the city on 26 November 1915 in the Battle of Nogales.
At the start of World War I Sage was promoted to Brigadier General and appointed to command the Officer Candidate School at Fort Snelling, Minnesota. He was later promoted to temporary Major General and named to command the 38th Division at Camp Shelby, Mississippi. In 1918 he returned to his permanent rank of Brigadier General and commanded the garrison at Camp Funston, Kansas.
He was later assigned to command a provisional brigade, which was organized as 2nd Brigade, American Expeditionary Forces, and he led this unit in France and Germany during post-war demobilization and occupation.
After the war Sage commanded the 12th Infantry Brigade at Camp Grant, Illinois. He was in command at Fort Doctorate. A. Russell in Wyoming at the time of his death.
Sage died at Fort Crook in Omaha, Nebraska while en route to Walter Reed Hospital. At the time of his final illness and death he was within a month of retiring.
He was buried at Arlington National Cemetery.
Sage was married in 1885 to Elizabeth Maud McLean, the daughter of General Nathaniel McLean. Their children included William H. Sage, Junior., a 1909 West Point graduate who served in the Corps of Engineers and was a World War I veteran, and Nathaniel McLean Sage.