Major Gregory W. Mateo was born in Berkeley, California and graduated from the United States Naval Academy in May of 2012. Following graduation from The Basic School, the Infantry Officers Course, and the Light Armored Reconnaissance Leaders Course, Major Mateo reported to 3d Light Armored Reconnaissance Battalion as a platoon commander. In October of 2014, Major Mateo deployed as the LAR platoon commander for BLT 2/4 on the 31st Marine Expeditionary Unit. 
 
In December 2015, Major Mateo reported to I Marine Expeditionary Force to begin the pre-deployment workup with Task Force Al Asad. Following graduation from Foreign Weapons Instructor Course, Major Mateo deployed to Al Asad Air Base in support of Operation Inherent Resolve where he served as the infantry tactics advisor and task force security section leader. Following re-deployment to California, Major Mateo served as the assistant operations officer at 3d Light Armored Reconnaissance Battalion. 
 
In July of 2017, Major Mateo reported to the Naval Postgraduate School in Monterey, California. While at NPS, his research focused on modeling and simulation analysis of emerging small unmanned aerial vehicle threats, weaponized opioids, and the effectiveness of friendly counter-systems and mitigation methods. He graduated in June of 2019 with a Master's Degree in Science in Operations Research. 
 
In July of 2019, Major Mateo reported to 2d Battalion, 5th Marines in Camp Pendleton, California. He served as the assistant operations officer from July of 2019 until April of 2020. Major Mateo served as the battalion operations officer while 2d Battalion, 5th Marines was deployed as the ground combat element of SPMAGTF-CR-CC 20.2 from May to October of 2020. While deployed, the battalion supported multiple operations and exercises across seven countries. 
 
In November of 2020, Major Mateo reported to 3d Battalion, 5th Marines and assumed command of Weapons Company. Following the completion of the battalion's pre-deployment workup, he deployed to Okinawa, Japan as part of the 31st Marine Expeditionary Unit.  While deployed, the 31st Marine Expeditionary Unit supported multiple theater security cooperation events, as well as service-directed experimentation training. 
 
Following his assignments in 5th Marine Regiment, Major Mateo reported to I Marine Expeditionary Force Expeditionary Operations Training Group in January of 2022. During this assignment, he served as the assistant operations officer and amphibious raids branch officer-in-charge. 
 
Major Mateo's personal decorations include the Navy and Marine Corps Commendation Award with gold star and the Navy and Marine Corps Achievement Medal with gold star.