Sergeant Major Mullen enlisted in the United States Marine Corps in December of 2003 and graduated from Marine Corps Recruit Depot Parris Island, South Carolina in 2004. Upon graduation, he received further training at Marine Combat Training Battalion, Camp Geiger, North Carolina, and Fort Sill, Oklahoma, he received the Military Occupational Specialty (MOS) of 0811, Field Artillery Cannoneer.
In 2005, Private First Class Mullen was assigned to Battery L, 3d Battalion, 10th Marine Regiment. From 2005 to 2008 Battery L was subject to rapid deployments in a provisional rifle company role. During this time, he served in the billets of cannon crewman, Prison guard, rifleman and fire team leader with three combat deployments to Camp Ar-Ramadi and Camp Fallujah, Iraq. In 2008 Corporal Mullen was promoted to the rank of Sergeant.
In 2008, Sergeant Mullen reported to The Basic School (TBS) in Quantico, Virginia. While assigned to TBS, he served as the Artillery Instructor Battery Platoon Sergeant and Battery Gunnery Sergeant. In 2009 Sergeant Mullen was selected as a Physical Training Instructor (PTI) and was assigned to Charlie Company, Officer Candidate School as a PTI. Upon returning to TBS, Sergeant Mullen was assigned to the Delta company staff and assigned as an instructor for newly commissioned officers in urban engagement, fire support, weapons and tactics.
In 2010, Sergeant Mullen was reassigned by request to return Battery L, 3d Battalion, 10th Marine Regiment to execute new equipment fielding and deployment with the M327 Mortar as part of the Expeditionary Fire Support System integration assigned to the 22D Marine Expeditionary Unit (MEU). In February 2011, the 22D MEU was activated early in support of NATO Operation Unified Protector in Libya until their return in March of 2012.
In the fall of 2012, Sergeant Mullen was assigned to Marine Forces Central Command Forward in the Kingdom of Bahrain. While serving at this assignment, Sergeant Mullen executed the responsibilities and duties as the Theatre Security Cooperation Chief. While in this role he was tasked with the designing, training, and instruction of bilateral training programs between U.S. Marine forces and regional partners within CENTCOM, with additional augmentations to Marine Embassy Beirut, Lebanon and the 1st Battalion, 507th Parachute Infantry Regiment as an advisor. In May of 2014, Staff Sergeant Mullen augmented deployments as the Joint Operations Chief for assignments in Afghanistan, Iraq, and Syria combating ISIL and ISIS threats while supporting Operations Enduring Freedom and Inherent Resolve, respectively.
In 2014, Staff Sergeant Mullen was assigned to Battery Q, 5th Battalion, 11th Marines in Camp Pendleton, California. While assigned to Battery Q, Staff Sergeant Mullen served as a High Mobility Artillery Rocket System (HIMARS) Section Chief and Platoon Sergeant. In 2015, Staff Sergeant Mullen led a detachment ISO Special Operations Command Upon off the coast of Somalia. Staff Sergeant Mullen was reassigned to Headquarters Battery, 5th Battalion, 11th and assumed the role of Headquarters Battery Gunnery Sergeant. In January of 2018, Gunnery Sergeant Mullen was reassigned to Battery R to execute a unit deployment to Okinawa serving as the Battery Gunnery Sergeant. Staff Sergeant Mullen was promoted to the rank of Gunnery Sergeant in July of 2018.
Gunnery Sergeant Mullen was assigned to Headquarters Marine Corps, Plans, Policies and Operations (PP&O), aboard the Pentagon in Washington D.C. in March of 2019. While assigned to PP&O, Gunnery Sergeant Mullen served as the Marine Corps Current Operations Chief, responsible for providing real-time force tracking, reporting, and daily briefings on all forward deployed Marine equities to the Commandant of the Marine Corps, Chairman of the Joint Chiefs of Staff, Secretary of Defense, U.S. Congress, and all general and executive levels of leadership within the Department of Defense.
In October of 2020, Gunnery Sergeant Mullen was selected by the Marine Corps Personnel Exchange Program to serve as the United States Marine Corps Exchange Staff Noncommissioned Officer for Physical Fitness and Combatives development, assigned to Spec Wing, Commando Training Centre-Royal Marines, in Lympstone, United Kingdom.
In September of 2022 Gunnery Sergeant Mullen was promoted to the rank of First Sergeant as he executed orders to 9th Communication Battalion, Camp Pendleton, California. Where he has served as the Unit Senior Enlisted Leader for HQ-Co I-MIG, Company A, Service Company, and Defensive Cyberspace Operations-Internal Defensive Measures Company.
He was promoted to the rank of Sergeant Major in April of 2025 and assumed his current duties as the Command Senior Enlisted Leader for 1st Air Naval Gunfire Liaison Company.
Sergeant Major Mullen's personal decorations include the Meritorious Service Medal, Joint Service Commendation Medal (2nd award), Navy and Marine Corps Commendation Medal (4th award) and the Navy and Marine Corps Achievement Medal (3rd award). Sergeant Major Mullen is a graduate of the All-Arms Commando Course, is a 4th Degree Blackbelt in the Marine Corps Martial Arts Program and holds a Master of Science degree in Homeland Security from California State University (Long-Beach).