Major General Mullen was commissioned via the NROTC program at Marquette University in 1986 and assigned to 1st Battalion, 3rd Marines where he served as a rifle and weapons platoon commander, and Battalion training officer from 1987 to 1990.
From 1990-1993 he served as a platoon commander, executive officer and Commanding Officer of the Fleet Anti-Terrorism Security Team Company, deployed to Operation Desert Shield and participated in several counter-narcotics missions in support of Joint Task Force-6, also serving as Battalion training officer.
In 1993, he was transferred to 2nd Battalion, 4th Marines (later converted to 2nd Battalion, 6th Marines), as the Commanding Officer of Fox Company. He participated in Operation Sea Signal in 1994, and deployed with the 26th Marine Expeditionary Unit from 1995 to 1996 as the small boat raid and cliff assault company commander. From 1996 to 1999 he served as the Inspector-Instructor for Fox Company, 2nd Battalion, 24th Marines and led a successful counter-narcotics mission in the Los Padres National Forest of California.
In 1999 he was selected to be the Marine Aide to the President, and served in that capacity until attending the School of Advanced Warfighting in 2001. After school, he reported to the Joint Operations Division of the Joint Staff, J-3 for duty in the PACOM and CENTCOM sections, and served as Executive Assistant to the Deputy Director for Regional Operations until 2004.
In 2004 he reported to 2nd Marine Division for duty as the Division Plans Officer, and was chosen later that year to be the Operations Officer for Regimental Combat Team - 8, with whom he deployed to Fallujah, Iraq from 2005 to 2006. After returning, he assumed command of 2nd Battalion, 6th Marines and returned to Fallujah in 2007. Following command he attended the Naval War College from 2008 to 2009, and subsequently commanded the Marine Corps Tactics and Operations Group from 2009 to 2012.
After selection to Brigadier General in 2012, he was assigned as the Commanding General, Education Command, and President, Marine Corps University. In 2013 he was assigned as the Director of Capabilities Development Directorate, Combat Development and Integration, then in 2015 he deployed and served as the Director of Operations and Target Engagement Authority in the Combined Joint Operations Center in Baghdad until June 2016. In July of 2016, he assumed command of Marine Corps Air Ground Task Force Training Command, Marine Corps Air Ground Combat Center and was promoted to Major General in July 2017.
Major General Mullen holds a BA and MA in Political Science from Marquette University, as well as an MA in National Security and Strategic Studies from the Naval War College. He is a graduate of the Advanced Artillery Officer Course, School of Advanced Warfighting, Airborne, Ranger, Summer Mountain Leader and Royal Marine Arctic Warfare Survival courses. He also co-authored the book Fallujah Redux which came out in 2014. All proceeds for the book go to the Semper Fi Fund. He and his wife Vicki have three adult children.