Sergeant Major St. John enlisted in the Marine Corps on 22 June 1998 and graduated from 1st Battalion, Marine Corps Recruit Depot, Parris Island, SC in September 1998. Following his successful completion of recruit training, he completed Marine Combat training at Camp Lejeune, NC, in November 1998, and further honed his skills by completing the Administrative Specialist course in December 1998.
From December 1998 to December 1999, Private St. John served as an Administrative Specialist at the Consolidated Administration Center, 3rd Force Service Support Group, Camp Kinser, Okinawa, Japan. It was during this tour that he achieved promotions to Private First Class and Lance Corporal.
In January 1999, Lance Corporal St. John was assigned to Marine Wing Support Squadron 273 (MWSS-273), Marine Aircraft Group 31, 2nd Marine Air Wing, MCAS Beaufort, SC. Initially serving as an Administrative Specialist, he eventually assumed the role of Legal Chief. His remarkable contributions led to his meritorious promotions to Corporal and Sergeant while serving with MWSS-273.
In May 2002, Sergeant St. John was assigned to Marine Light Attack Helicopter Squadron 773 (HMLA-773), Marine Aircraft Group 42, 4th Marine Air Wing, NAS Atlanta, Marietta, GA. As an Administrative Specialist and Legal Chief, he continued to excel, earning promotion to Staff Sergeant in March 2005. During his tenure with HMLA-773, he was attached to Marine Aircraft Group 29, 2nd Marine Aircraft Wing, Camp Lejeune, NC, and deployed to Al Asad, Iraq in support of Operation Iraqi Freedom from September 2007 to March 2008. Upon his return, he resumed his duties as an Administrative Chief at MAG-42, NAS Atlanta, until his honorable discharge in March 2009. He then transferred to the Individual Ready Reserve until July 2011.
In July 2011, Staff Sergeant St. John embarked on a new chapter in his military career by joining the Selected Marine Corps Reserves. He was assigned to Individual Mobilization Augmentee, Marine Forces Reserve, New Orleans, LA, assuming the position of Administrative Chief. Recognizing his outstanding performance, he was promoted to Gunnery Sergeant in April 2014 and transferred to the Mobilization Training Unit as an Administrative Inspector for the Force Readiness Activation Assessment Program.
In June 2016 Gunnery Sergeant St. John joined Combat Logistics Regiment 45 (CLR-45), 4th Marine Logistics Group, Marietta, GA, serving as an Administrative Chief. During his tenure, he participated in a deployment to Central America with the Special Purpose Marine Air-Ground Task Force, Southern Command 18, supporting Marine Forces South, Southern Command, Doral, FL from June 2018 to November 2018. He returned to CLR-45 in December 2018, resuming his role as an Administrative Chief.
In May 2019, Gunnery Sergeant St. John's exceptional leadership abilities and dedication were recognized once again as he was frocked to First Sergeant. He assumed the responsibilities of Company First Sergeant for Headquarters and Service Company, 3rd Battalion, 25th Marine Regiment, 4th Marine Division, Brook Park, OH. Continuing his remarkable career progression, in May 2022, First Sergeant St. John was transferred to Defense Cyberspace Operations-Internal Defensive Measure Company Bravo, 6th Communications Battalion, Force Headquarters Group, Ayer, MA, where he served as the Company First Sergeant.
In April 2023, First Sergeant St. John was frocked to the rank of Sergeant Major. He was subsequently assigned to Marine Wing Support Squadron 472 and assumed the duties of Squadron Sergeant Major on 22 July 2023.
Sergeant Major St. John's remarkable contributions and commitment have earned him several personal awards, including the Navy and Marine Corps Commendation Medal with two gold stars, Navy and Marine Corps Achievement Medal with two gold stars, Iraq Campaign Medal, and Humanitarian Service Medal.