Ennis was born on April 27, 1904, at
Fort Hamilton, New York, where his father, Lieutenant (later brigadier general)
William P. Ennis Sr. (1878–1968), was stationed. His mother was Eda (Totten) Ennis. He was a direct descendant of Lieutenant William Ennis who served in the
Continental Army during the
American Revolution and was an original member of the Rhode Island
Society of the Cincinnati. He entered the
United States Military Academy at
West Point, New York, in 1922 and graduated in 1926. He had strong family ties to West Point as both his father and grandfather, Brigadier General
William Ennis, were graduates of West Point. Like his father and grandfather, Ennis was commissioned as an Artillery officer and rose to become a general. On his mother's side of the family, he was the grandson of 1st Lieutenant
C.A.L. Totten and the great-grandson of Brevet Brigadier General
James Totten, both of whom were also graduates of West Point. ==Military career==