Sergeant Major Christopher Demosthenous enlisted in the Marine Corps on 9 September 1996. After completing Recruit Training at MCRD Parris Island, SC, he reported to the School of Infantry East for Marine Combat Training (MCT). Upon completion of MCT, he reported to Fort Sill, Ok for his Military Occupational Specialty at the Field Artillery School to become a Cannon Crewman. Once his training was complete he was assigned to 3rd Battalion 12th Marines in Okinawa, Japan. During this period he was promoted to Lance Corporal.
In January of 1998, Sergeant Major Demosthenous received orders to Lima Battery 3rd Battalion 11th Marines, in 29 Palms, Ca. In April of 1998, he was promoted to Corporal and assigned the billet of Section Chief. In July of 1998 and September 1999, he deployed with Lima Battery on a UDP to Okinawa, Japan.
In June 2000 Sergeant Major Demosthenous received orders to attend Marine Security Guard School in Quantico, Va where he was promoted to the rank of Sergeant in July 2000. After completion of Marine Security Guard School, he was assigned to Lusaka, Zambia as his first post, and was re-assigned to Ottawa, Canada for his second post.
In March of 2003, Sergeant Major Demosthenous received orders to Lima Battery 3rd Battalion 11th Marines in 29 Palms, Ca. He was promoted to Staff Sergeant in February 2004, and was then assigned to the billet of Platoon Sergeant for Guns Platoon. In March 2004, he deployed to Ar Ramadi, Iraq in support of Operation Iraqi Freedom. In September 2005, he deployed on a UDP to Okinawa, Japan, then in September 2006, he deployed to Rawah, Iraq, where he was attached to 2nd LAR and served as Platoon Commander and Quick Reaction Force Team Commander.
In October 2007, Sergeant Major Demosthenous received orders to attend Drill Instructor School in Parris Island, SC. After completion of Drill Instructor School he attended Marine Combat Instructor of Water Survival School (MCIWS). After completion of MCIWS he was assigned to Golf Company, 2nd Recruit Training Battalion where he did eight training cycles and held the billets of drill instructor, senior drill instructor and series chief drill instructor. In July 2009, he was promoted to the rank of Gunnery Sergeant.
In November 2010, Sergeant Major Demosthenous received orders to 1st Battalion, 11th Marines in Camp Pendleton, Ca where he was assigned the billet of Battery Gunnery Sergeant for India Battery 3rd Battalion, 12th Marines. In May 2012, he deployed with the unit on the 31st MEU.
In January 2013, he was selected to the rank of First Sergeant and received orders to MCRD San Diego, Ca. While at MCRD San Diego he held the billet of Company First Sergeant for Charlie Company, 1st Recruit Training Battalion for eight training cycles. In June 2015, Sergeant Major Demosthenous received orders to Marine Corps Base Quantico, Va, where he held the billet of Company First Sergeant for Tenant Activities Company and two augment cycles with Officer Candidates School.
In May 2018, he was promoted to the rank of Sergeant Major and reported to Chicopee, Ma where he held the billet of Site Sergeant Major for Site Support Chicopee, MACG-48. In May 2021, Sergeant Major Demosthenous reported to Camp Lejeune, N.C. where he held the billet of Battalion Sergeant Major for II MEF Support Battalion. In March of 2023, Sergeant Major Demosthenous reported to Camp Pendleton, Ca where held the billet of Regimental Sergeant Major for 11th Marine Regiment. In June of 2025, Sergeant Major Demosthenous received orders to Headquarters Battalion, 1st Marine Division where he currently holds the billet of Command Senior Enlisted Leader.