Ennis was born in the Presidio of San Francisco on January 30, 1878, though he spent much of his early life in Newport, Rhode Island.

He was promoted to colonel in May 1918 and served as the director of the Department of Materiel, for which he received the Army Distinguished Service Medal, the citation for which 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 Peirce Ennis, 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.

As Director of the Department of Material, School of Fire, for Field Artillery at Fort Sill, Oklahoma, from 9 May 1918 to 30 August 1918, General Ennis' untiring energy, devotion to duty, and exceptional qualifications for the task he had to perform, were directly responsible for the splendid organization developed.

[1][2]After Ennis's promotion to the rank of brigadier general on August 8, 1918, he assumed command over the 13th Field Artillery Brigade, 13th Division, stationed at Fort Lewis.

[3] General Ennis was an hereditary companion of the District of Columbia Commandery of the Military Order of the Loyal Legion of the United States.