A native of Newport, Rhode Island, Ennis was educated in local public and private schools before attending the United States Military Academy at West Point, from which he graduated in 1864.

Commissioned a second lieutenant of Field Artillery, Ennis served in the Union Army at the end of the American Civil War and participated in campaigns and battles in Tennessee and Alabama, in addition to the Defenses of Washington.

[1] He was educated in the public and private schools of Newport, and began attendance at the United States Military Academy (West Point) in 1860.

[4] Many years later, Ennis stated in an interview that MacLay and he saw Booth run across the stage, but were not immediately aware Lincoln had been shot.

[4] Following the end of the Civil War, Ennis served with his regiment at Fort Brown near Brownsville, Texas from June 1865 to January 1866.

[1] While participating in a skirmish against the Bannocks in Idaho, he was temporarily blinded by alkali dust, which required him to return to the Presidio to recuperate.

[3] At the start of the Spanish–American War, Ennis was assigned to Tampa, Florida as commander of a battalion of siege artillery that was part of Fifth Army Corps.

[3] In February 1899, Ennis testified at the military court of inquiry that investigated the United States Army beef scandal.

[6] According to Ennis, he heard no complaints about the canned beef issued to the army during the war, but often found that he could not eat himself because it left him nauseated.

[6] Upon making inquiries of soldiers after their return to the United States, Ennis found that they refused to eat the canned beef because it caused them to vomit.

[3] Ennis served in the office of the Adjutant General of the United States Army from August 1902 to April 1903, when he was promoted to colonel.

[3] On November 7, 1905, he was promoted to brigadier general under the provisions of a law permitting Union Army veterans still on active duty to be advanced one grade.

[1] In addition to membership in the Military Order of the Loyal Legion of the United States, Ennis subscribed to the Newport Reading Room and was a member of the West Point Mess and Fort Monroe Club.

[1] In 1933, he broke his hip in a fall from his car; given his age, physicians feared the injury would prove fatal, but Ennis made a full recovery.

[1] In 1926, the U.S. Congress enacted a law permitting West Point and United States Naval Academy graduates to be awarded Bachelor of Science (BS) degrees.

[7] As a result, Ennis was awarded his BS at his home by Colonel Clement H. Wright, an Army officer then serving as an instructor at the Naval War College in Newport.