Sgt Maj John Coyne enlisted in the Marine Corps in March 1990, and underwent recruit training at First Recruit Training Battalion, Alpha Company, Marine Corps Recruit Depot, Parris Island, South Carolina. Private First Class Coyne graduated recruit training in October 1990 and then attended Basic Electrician School after completing Marine Combat Training.
Lance Corporal Coyne graduated from Utilities Instruction Company, Marine Corps Engineer School, Courthouse Bay, Camp Lejeune, North Carolina as the Honor Graduate, and was meritoriously promoted. He was then ordered to the Third Marine Aircraft Wing, Marine Corps Air Station El Toro, California, where he served in numerous billets with Marine Wing Communications Squadron 38, Headquarters and Headquarters Service Squadron 38, which was later redesignated as Marine Tactical Air Command Squadron 38, and Marine Air Traffic Control Squadron 38. Corporal Coyne was meritoriously promoted in June 1992, and deployed later that year to Operation Restore Hope in Somalia, Africa.
After being promoted in March 1994, Sergeant Coyne reenlisted and was assigned to 3rd Supply Battalion, 3rd Force Service Support Group, Okinawa, Japan. In 1995, Sergeant Coyne was ordered to Marine Corps Recruit Depot, Parris Island, South Carolina, to serve with Headquarters and Service Battalion. During this time he attended the Journeyman Electrician School at Marine Corps Engineer School, Courthouse Bay, Camp Lejeune, North Carolina.
In 1997 Sergeant Coyne transferred from active duty to the Selected Marine Corps Reserve, and joined Marine Wing Support Squadron 472, 4th Marine Aircraft Wing in Marietta, Georgia. Staff Sergeant Coyne transferred to Headquarters Company, 4th Maintenance Battalion, 4th Marine Logistics Group in Charlotte, North Carolina in December, 2001 after MWSS-472 was disbanded. There he served as the Utilities Chief. In January 2003, Staff Sergeant Coyne was mobilized under Presidential Recall for Operation Iraqi Freedom, and served as Company Gunnery Sergeant for 8th Engineer Support Battalion. He also attended the Utilities Chief Course at Marine Corps Engineer School, Courthouse Bay, Camp Lejeune, North Carolina, graduating as the Honor Graduate.
In 2005, Staff Sergeant Coyne volunteered for mobilization under Presidential Recall, and was deployed to Anbar Provence to support Operation Iraqi Freedom, serving with Combat Logistics Regiment 25. In May 2006, Gunnery Sergeant Coyne returned to 4th Maintenance Battalion in Charlotte, North Carolina, to serve as the Utilities Chief.
In March 2010 Gunnery Sergeant Coyne joined Headquarters Company, Headquarters and Service Battalion, 4th Marine Logistics Group in Marietta, Georgia, where he served as Company Gunnery Sergeant.
In June 2012, First Sergeant Coyne assumed the post of Battery First Sergeant for Battery M, 3d Battalion, 14th Marine Regiment, 4th Marine Division, in Chattanooga, Tennessee. In June 2016, First Sergeant Coyne assumed the post of Battery First Sergeant for Battery K, 2d Battalion, 14th Marine Regiment, 4th Marine Division, in Huntsville, Alabama.
Sergeant Major Coyne assumed his current post as Sergeant Major of HMLA-773, MAG-49, 4th Marine Air Wing in June 2017. His personal awards include the Meritorious Service Medal, the Navy and Marine Corps Commendation Medal, and the Navy and Marine Corps Achievement Medal with two stars.