1stSgt Mullen began his career in 2002, enlisting in the Marine Corps as an infantryman with the goal of becoming a Reconnaissance Marine. After completing training in San Diego, he was assigned to the 3rd Light Armored Reconnaissance Battalion, where he deployed to Operation Iraqi Freedom. There, he manned the notoriously deadly 25mm chain gun during battles in Fallujah and Ramadi.
In 2005, he was promoted to Corporal and deployed with 2/5 WPNS Company on the 31st Marine Expeditionary Unit throughout Southeast Asia, serving as a Vehicle Commander and Section Leader. In late 2006, he transitioned to the Select Marine Corps Reserve.
In 2009, Mullen was promoted to Sergeant and attended the Basic Reconnaissance Course, after which he joined the 4th Recon Battalion. He served as a Platoon Sergeant before being promoted to Staff Sergeant and deploying to Afghanistan in 2011, where he embedded with the Afghan Army as an advisor.
From 2012 to 2017, he served as a Platoon Sergeant and Operations Chief for Charlie Company, 4th Recon Battalion, in San Antonio.
In 2017, Mullen was promoted to Gunnery Sergeant and took on the role of South Texas Regional Readiness Chief for the Individual Ready Reserve.
In 2021, he became the Operations Chief and Fires Support Coordination Chief for WPNS Company, 1/23 Marines in Austin.
In late 2022, Gunnery Sergeant Mullen was selected to join the new Marine Innovation Unit. Shortly thereafter, he was selected for the rank of 1stSgt and rejoined WPNS Company, 1/23 Marines as the Senior Enlisted Advisor and Company 1stSgt.
Throughout his 20-year career in the Marine Corps, 1stSgt Mullen has gained proficiency as a rifleman, machine gunner, trained observer, and communicator. He has also become a military free-fall jumpmaster, mentored foreign militaries, and trained in fires support coordination. He has fought in two conflicts and supported humanitarian operations both in Texas and across the globe.