He graduated from the United States Military Academy in June 1915 (as part of "the class the stars fell on") and was commissioned a second lieutenant of Infantry.

He was an observer at the front with the 4th French Army at Chalons-sur-Marne for a week before becoming director of the 2d Corps Aeronautical School in February and March 1918.

In April 1919 McNarney was attached to the American Expeditionary Force Headquarters in Paris to write a manual on air observation and was promoted to lieutenant colonel in May.

McNarney experienced the chaotic ups and downs in rank common to Regular officers following the reorganization of the Army by the National Defense Act of 1920.

McNarney was an instructor at the Army War College in Washington from August 1933 to March 1935, when he went to Langley Field, Virginia, as G-4, helping in the organization of the new General Headquarters Air Force with immediate promotion to lieutenant colonel.

After the attack on Pearl Harbor McNarney served on the Roberts Commission which investigated the Army and Navy commanders in Hawaii.

He succeeded William Bryden as Marshall's deputy chief of staff in March with promotion to lieutenant general in June.

While deputy chief of staff, McNarney developed a plan for land-based anti-submarine warfare under which the AAF organized the Army Air Forces Antisubmarine Command with a mission to attack hostile submarines "wherever they may be operating".

This offensive measure materially aided destruction of the German hold on sea lanes before it was disbanded in 1943 as redundant to efforts of the United States Navy.

In October 1942, AAF Commanding General Henry H. Arnold proposed to Marshall that an Army man be named Supreme Allied Commander for the war effort in the Pacific and suggested Douglas MacArthur, McNarney or Lesley McNair for the position, Marshall without comment passed the request to his staff for analysis.

Streett, an airman himself, was also in favor of a supreme commander but he recognized the political challenges, projecting that the president would have to make the appointment, not a committee of military men.

Streett suggested McNarney or Admiral Chester W. Nimitz, depending on whether an air or a naval strategy was considered most important.

On 15 June 1946, Clay, frustrated with the difficulty of the job given his position, wrote to Secretary of State James Byrnes requesting his retirement.