Sean J. Grantham attended recruit training aboard Marine Corps Recruit Depot San Diego in August 2001. Private First Class Grantham reported to Tactical Air Defense Controller Course aboard MCAGCC 29 Palms, CA in December 2001 and later reported for duty with Marine Air Control Squadron-1 (MACS-1), MCAS Yuma, AZ in May 2002.

While assigned to MACS-1, Private First Class Grantham was promoted through the rank of Sergeant and deployed three times in support of Operation Iraqi Freedom between January 2003 and January 2006. In January 2008 Sergeant Grantham reported to Marine Air Command and Control Squadron Experimental (MACCS-X)as the Air Defense Division Chief. He was promoted to Staff Sergeant in July 2008 and received orders to recruiting duty.

Staff Sergeant Grantham reported to RS Orange in January 2011 and served as a production recruiter in Upland, CA and RSS SNCOIC in Anaheim, CA. Staff Sergeant Grantham was promoted to Gunnery Sergeant in October 2013 and received orders to MACS-1, Early Warning and Control (EW/C) Detachment, MCAS Miramar.

Gunnery Sergeant Grantham deployed in July 2014 to Qatar and the United Arab Emirates in support of Operation Inherent Resolve, Operation Enduring Freedom, and the Combined Defense of the Arabian Gulf. Upon returning from deployment, Gunnery Sergeant Grantham assumed the Detachment Senior Enlisted Advisor billet. Gunnery Sergeant Grantham received orders and reported to MACS-4, Okinawa Japan in September 2016 as the Detachment Senior Enlisted Advisor.

Gunnery Sergeant Grantham was promoted to First Sergeant and reported to 3d Maintenance Battalion, Camp Kinser, Okinawa in June 2017. While with Maintenance Battalion, First Sergeant Grantham served in General Support Maintenance Company and Ordnance Maintenance Company. After his tour in Okinawa, First Sergeant Grantham reported to Marine Combat Training Battalion, School of Infantry- West until his promotion to Sergeant Major. Sergeant Major Grantham was assigned to VMGR-152 in June 2022 and was later reassigned to Marine Aviation Logistics Squadron 12 in January 2024.

Sergeant Major Grantham's personal awards include the Meritorious Service Medal, Navy and Marine Corps Commendation Medal x 2, Navy and Marine Corps Achievement Medal x 2, and the Army Achievement Medal. He is married to Christina Grantham and they have one child, Xander Grantham.