Douglas E. Gardner enlisted in the Marine Corps in August of 2000 and completed recruit training at MCRD San Diego, CA in September of 2001. Following recruit training, Private Gardner reported to Marine Combat Training Battalion (Bn), School of Infantry (SOI) West, Camp Pendleton, CA. Upon completion in November of 2001, Private Gardner attended Military Police School, Marine Detachment Fort Leonard Wood, MO. In January of 2001, Private First Class Gardner graduated from Military Police School and reported to 2d Military Police (MP) Bn, 2d FSSG, in Camp Lejeune, NC.

In January 2003, Lance Corporal Gardner deployed with 2d MP Bn to Kuwait in support of Operation Iraqi Freedom. Lance Corporal Gardner returned from deployment in July of 2003, in order to begin pre-deployment training with the 22d MSSG, 22d MEU. In March of 2004, Corporal Gardner deployed to the Kandahar and Oruzgan provinces of Afghanistan, in direct support of Operation Mountain Storm. In September of 2004, Corporal Gardner returned from deployment and received a Combat Meritorious promotion to the rank of Sergeant.

In October of 2004, Sergeant Gardner returned to 2d MP Bn, and was assigned duties as the S-3 Training NCOIC. In August of 2005, Sergeant Gardner attached to 5th Battalion, 14th Marine Regiment, Provisional Military Police Battalion (-/REIN), and deployed to Al Asad, Iraq as a Criminal Investigator.

Sergeant Gardner returned to 2d MP Bn in April of 2006, and shortly thereafter, received orders to the Criminal Investigation Division, Provost Marshal's Office, MCB Camp Lejeune, NC. In January of 2007, Sergeant Gardner attended the U.S. Army Apprentice Special Agent Course and inherited the 5821 Military Occupational Specialty.

In June of 2008, Staff Sergeant Gardner executed orders to the Criminal Investigation Division, Provost Marshal's Office, Headquarters and Headquarters Squadron, MCAS Miramar, CA. In December of 2009, Staff Sergeant Gardner was selected for assignment as a Marine Special Agent with the Naval Criminal Investigative Service, Marine West Field Office, Camp Pendleton, CA where he served as a member of the Family and Sexual Violence squad and Major Case Response Team. In November of 2011, Staff Sergeant Gardner deployed to Camp Leatherneck, Afghanistan in support of the NCIS mission. Staff Sergeant Gardner returned from deployment in March of 2012.

In July of 2012, Staff Sergeant Gardner attended the Marine Combat Instructor School, SOI West, and was subsequently assigned to Fox Company, Marine Combat Training Bn, SOI West, as a Squad Instructor. In May of 2013, Staff Sergeant Gardner transferred to Advanced Infantry Training Battalion, SOI West to instruct under the Combat Hunter Program, and in September of the same year, was appointed as the Chief Instructor/SNCOIC.

In July of 2015, Gunnery Sergeant Gardner executed orders to the Criminal Investigation Division, Provost Marshal's Office, MCIPAC-MCB Camp Butler, Okinawa, Japan, to serve in the capacity of District Chief Investigator. In April of 2016, Gunnery Sergeant Gardner was frocked to the rank of First Sergeant. In May of 2017, First Sergeant Gardner executed orders to Military Police Company C, 3D Law Enforcement Bn, III Marine Expeditionary Force (MEF) Information Group, III MEF where, within a one year period, his company deployed to the Philippines, Mongolia, and South Korea in an effort to support the III MEF theater security cooperation strategy.

In May of 2018, First Sergeant Gardner executed orders to Assault Amphibian School, Training Command, Camp Pendleton, CA, where he served in the capacity of Company First Sergeant for both Training and Instructor Company and Headquarters and Support Company.

In November of 2021, First Sergeant Gardner was frocked to his current rank and in December, was assigned duties as the Battalion Sergeant Major of 3rd Battalion, 1st Marines, 1st Marine Division. His personal decorations include the Navy and Marine Corps Commendation Medal with three gold stars in lieu of fourth award, the Navy and Marine Corps Achievement Medal with three gold stars in lieu of fourth award, and the Army Achievement Medal.