Mills is a retired United States Marine Corps officer, who served as commander of the Marine Forces Reserve from 2013 to 2015 and Marine Forces North, headquartered at the Marine Corps Support Facility New Orleans.
Previously he was Deputy Commandant for Combat Development and Integration and Commanding General, Marine Corps Combat Development Command in Quantico, Virginia, and, prior to that assignment, was commander of the I Marine Expeditionary Force (Forward)/Regional Command Southwest in Afghanistan.