A West Point graduate who learned to fly in 1915–16, he served with the 1st Aero Squadron in the Pancho Villa Expedition and later led it on the Western Front.

During World War II as a brigadier general, he led the Royce special mission to Mindanao, in which a small force of bombers flew from Australia to attack Japanese targets in the Philippines.

Among his fellow graduates were several men who would reach general officer rank in the future, such as Carl Spaatz, Brehon B. Somervell, Frank W. Milburn, Harold R. Bull, John B. Anderson, James L. Bradley, Charles P. Gross, Jens A. Doe, Orlando Ward, Harold Francis Loomis, Vicente Lim, and Harry C. Ingles.

[1] For his service in France, he was awarded the Croix de Guerre with the citation: "Commanding the 1st American Observation Escadrille, he insisted on making the first reconnaissance above the enemy lines himself.

"[6] Royce experienced the chaotic ups and downs in rank common to Regular officers in 1920, when the National Defense Act of 1920 reorganized the military.

[1] In January 1930 Royce commanded the 1st Pursuit Group in a flight across the northern United States from Selfridge Field to Spokane, Washington and back in what became known as the "Arctic Patrol" flight, in order to put "pursuit tactics to the acid test under extremely rigorous weather conditions, and to afford a very broad opportunity for testing flying equipment in zero temperatures".

In February 1935 he led the 1st Pursuit Group in a series of a cold weather flights through the northern states where the airmen encountered blizzards, and subzero temperatures.

He assumed command of the 20th Bombardment Wing at Fort Douglas, Utah in March 1941 and was promoted to brigadier general in April.

Royce inexplicably declined to disperse the B-17s, and one was damaged by an air attack on Del Monte during the subsequent mission after it had been left behind for repairs.

Royce returned to the United States in September 1942 and assumed command of the South Eastern Training Center at Maxwell Field in Montgomery, Alabama.

He commanded the 1st Tactical Air Force (Provisional) from October 1944 to January 1945, operating in support of the Sixth United States Army Group.

[1] Royce was married again in Detroit in February 1945, this time to Agnes Berges an ex-Manhattan hotel executive and overseas Red Cross worker.