Sgt. Maj. Marshall H. Gregg enlisted in the United States Marine Corps in June 1998. He attended recruit training at Parris Island, South Carolina with follow on training at Infantry Training Battalion, School of Infantry-East where he obtained the military occupational specialty (MOS) of 0311, Basic Rifleman. Follow-on training was conducted at Marine Security Force School, Chesapeake, Virginia, where he obtained the MOS of 8152, Marine Corps Security Force (MCSF) Guard.  
 
In 1999, Pfc. Gregg reported to Second Fleet Anti-terrorism Security Team, Yorktown, Virginia. He deployed twice to Naples, Italy in support of Marine Corps Security Mission Europe, to Kosovo in support of Operation JOINT GUARDIAN, and to Macedonia to quell riots being conducted on the U.S. Embassy.
 
In 2002, Cpl. Gregg reported to Marine Barracks 8th and I, where he was promoted to Sergeant and served as Sergeant of the Guard for the security force.  
 
In 2003, Sgt. Gregg reported to 2d Battalion, 8th Marine Regiment and was deployed to Korengal, Afghanistan in support of Operation Enduring Freedom (OEF), to the Middle East with the 26th Marine Expeditionary Unit (Special Operations Capable) and to Fallujah and Rawah, Iraq in support of Operation Iraqi Freedom. During these tours, he served as Squad Leader, Element Leader, and Platoon Sergeant and was promoted to Staff Sergeant.
 
In 2007, Staff Sgt. Gregg reported to Marine Combat Instructor School and Formal School Instructor Course and was then assigned to School of Infantry-East where he served as Combat Instructor, Platoon Commander, Chief Combat Instructor, Master Instructor, Weapons Section Chief Instructor and Company Gunnery Sergeant. During this tour he was promoted to Gunnery Sergeant, meritoriously.  
 
In 2011, Gunnery Sgt. Gregg reported to Weapons Company, 3d Battalion, 4th Marines, 7th Marine Regiment, and served as CAAT Platoon Sergeant, Weapons Company Operations Chief, and Weapons Company First Sergeant. During this tour he deployed to Kajaki, Afghanistan in support of OEF and was promoted to First Sergeant.  
 
In 2014, 1st Sgt. Gregg reported to Marine Corps Logistics Command and assumed the duties of Headquarters Company-East First Sergeant. In 2015, he was assigned to the billet of Marine Corps Logistics Command Headquarters Group Senior Enlisted Advisor, advising the Group O-6 Commander.
 
In 2017, 1st Sgt. Gregg reported to Third Marine Raider Battalion, Marine Corps Forces, Special Operations Command to assume duties as Company First Sergeant for Marine Special Operations Company-L and was deployed to Somalia, Africa in support of Special Operations Command-East Africa and Operation Octave Shield. While at Third Marine Raider Battalion, he was promoted to his current rank.
 
In May 2019, Sgt. Maj. Gregg assumed duties as the Command Senior Enlisted Leader of Marine Light Attack Helicopter Training Squadron 303, 3rd Marine Aircraft Wing.
 
In January 2022, Sgt. Maj. Gregg assumed duties as the Command Senior Enlisted Leader of 2d Battalion, 11th Marines, 1st Marine Division.  
 
In April 2023, Sgt. Maj. Gregg assumed duties as the Command Senior Enlisted Leader of Headquarters and Support Battalion, MCB Camp Lejeune.

Sgt. Maj. Gregg's personal awards include the Meritorious Service Medal with two gold stars, Joint Service Commendation Medal, Navy and Marine Corps Commendation Medal with gold star, Joint Service Achievement Medal, Navy and Marine Corps Achievement Medal with two gold stars, and Combat Action Ribbon with gold star.