Sergeant Major Cuellar is a native of Texas, on 2 December 2002, he attended recruit training aboard Marine Corps Recruit Depot, San Diego, CA. Upon completion of recruit training, he attended Infantry Training Battalion in Camp Pendleton, CA for MOS school and designated as an 0311 where he was promoted to Private First Class. 

Upon graduation, he would receive orders to 2D Battalion, 3D Marines, Echo Company where he was later promoted to Lance Corporal. While serving with Echo Company, Sergeant Major Cuellar deployed to Okinawa, Japan where 2D Battalion, 3D Marines was assigned to the 31st Marine Expeditionary Unit (MEU). Sergeant Major Cuellar participated in various exercises throughout this deployment such as Foul Eagle (South Korea) and Cobra Gold (Thailand). 

During the work up for his second deployment he was promoted to the rank of Corporal on 1 June 2005. In June of 2005 he deployed to the Kunar Province, Afghanistan, upon his return from deployment, Sergeant Major Cuellar was reassigned to Third Platoon as a squad leader. 

On 1 June 2006, Sergeant Major Cuellar was promoted to the rank of Sergeant. Sergeant Major Cuellar would deploy to Hadithah, Iraq for his third deployment as an infantry squad leader and make his fourth deployment to Karmah, Iraq. 

Sergeant Major Cuellar deployed on his fifth deployment to Gulistan, Afghanistan in Helmand Province as a Scout Sniper Team Leader. On 1 November 2010, Sergeant Major Cuellar was promoted to the rank of Staff Sergeant. Sergeant Major Cuellar would go on his sixth deployment to Marjah, Afghanistan in Helmand Province as the Chief Scout and Team Leader for the Scout Sniper Platoon. 

After completing Recruiting School, Sergeant Major Cuellar was assigned to RS San Antonio, RSS Corpus Christi as a Canvassing Recruiter in February 2012. Sergeant Major Cuellar received PCA orders to RSS South Park as the stations SNCOIC in January of 2013. Sergeant Major Cuellar was meritoriously promoted to Gunnery Sergeant on 2 July 2014. 

Upon a successful tour on Recruiting Duty Sergeant Major Cuellar would receive orders to 3D battalion, 3D Marines and assigned as the Scout Sniper Platoon Sergeant. Shortly upon his arrival, he would deploy to Okinawa, Japan as part of the UDP-H rotation for his seventh deployment. Upon returning from UDP, Sergeant Major Cuellar was reassigned to Lima Company as the Company Gunnery Sergeant and would also assume the billet as the Company First Sergeant. 

Sergeant Major Cuellar received PCA orders aboard MCB Kanehoe Bay, Hawaii to 3D Marine Regiment, Combat Assault Company to serve as the Operations Chief. In the fall of 2017 Sergeant Major Sergeant Cuellar would receive his second iteration of SDA orders, this time to Combat Instructor duty, SOI-E aboard Camp Geiger, N.C. and be assigned to Infantry Training Battalion, Alpha Company as the Company Gunnery Sergeant. 

After a short stint on Combat Instructor Duty, Sergeant Major Cuellar was selected to the rank of First Sergeant and assigned to CLB-8 as the Company First Sergeant for Transportation and Services Company within 2d MLG. Shortly after his arrival to CLB-8, Sergeant Major Cuellar would be selected to go on the SPMAGTF-CR-AF 19.1 as the Senior Enlisted Advisor for the Logistics Combat Element. After an eighth deployment Sergeant Major Cuellar would return to CLB-8 and assume the responsibilities of H&S Company First Sergeant. 

In March 2020, Sergeant Major Cuellar would PCS to Okinawa, Japan and report to 3rd Intel Battalion and assume the responsibilities of Battlespace Surveillance Company First Sergeant. Sergeant Major Cuellar received PCA orders to 4th Marine Regiment to assume the duties as the H&S Company First Sergeant. Shortly after his arrival, Sergeant Major Cuellar was selected to his current rank and would receive PCA orders. 

Sergeant Major Cuellar assumed the duties and responsibilities as the III MEF Support Battalion Sergeant Major on 5 April 2022. Sergeant Major received PCS orders to Marine Corps Air Ground Combat Center aboard Twentynine Palms and assume the role of Battalion Sergeant Major for 3D Battalion, 7TH Marines on 16 March 2023. 

Sergeant Major Cuellar's personal decorations include the Meritorious Service Medal, Navy Commendation Medal (4th award), Navy and Marine Corps Achievement Medal's with Combat V (7th award), Combat Action Ribbon w/gold star. SgtMaj. Cuellar is married to MaryKae Cuellar and they have two children Leonidus and Leylani Cuellar and Sergeant Major Cuellar has an older son Leo Cuellar III.