The Assistant Commandant of the Marine Corps (ACMC) is the second highest ranking officer in the United States Marine Corps and serves as the second-in-command for the Commandant of the Marine Corps (CMC). Before the official title “Assistant Commandant of the Marine Corps” was adopted in 1946, the title of the position was known as “Assistant to the Commandant” and before 1918, known only as “Duty in the Office of the Commandant”. Few records exist detailing the specific duties of this billet prior to the outbreak of WWI because the Commandant of the Marine Corps had only an administrative staff and no deputy.  Since its establishment by Congress in 1946, 33 Marines have held the position of Assistant Commandant of the Marine Corps.

As with the Commandant, the Assistant Commandant of the Marine Corps is appointed by the President of the United States and must be confirmed by a Senate majority vote. In the event the Commandant is absent or unable to perform his duties, the Assistant Commandant assumes the duties and responsibilities of the Commandant and, with the approval from the Secretary of the Navy, may delegate or prescribe for him. Orders issued by the Assistant Commandant while performing such duties have the same weight as those issued by the Commandant.  For this reason, the Assistant Commandant is promoted to a rank equal to that of the sitting Commandant, a four-star general.

The first “Assistant to the Commandant” was Lieutenant Colonel Eli K. Cole who assumed the position on April 29, 1911. From this date until the adoption of the current position, 19 men were assigned to assist the Commandant in the performance of his duties. The first Marine designated as the Assistant Commandant of the Marine Corps, Major General Lemuel C. Shepherd, Jr. held the billet from October 17, 1946 to April 14, 1948. Since the position's inception in 1946, seven Assistant Commandants have moved on to become Commandant: Major General Lemuel C. Shepherd, Jr., Lieutenant General Randolph M. Pate, Lieutenant General Leonard F. Chapman, Jr., General Robert H. Barrow, General Paul. X. Kelley, General James F. Amos, and General Joseph F. Dunford, Jr.