Major David S. Mercer commissioned in the United States Marine Corps in July 2011 after graduating from the University of Kentucky. In March 2012, Second Lieutenant Mercer attended The Basic School at Quantico, VA, where he completed the Infantry Officer Course and graduated in March 2013.
Upon completion of The Basic School, Second Lieutenant Mercer reported to 2d Battalion, 7th Marines at Twenty-Nine Palms, CA, where he assumed command of 2nd Platoon, Golf Company. In 2014, he led the platoon on deployment with Special Purpose Marine Air-Ground Task Force-Crisis Response-CENTCOM to Jordan. After returning, 1st Lieutenant Mercer became the Scout Sniper Platoon Commander for 2d Battalion, 7th Marines. In December 2015, First Lieutenant Mercer assumed the role of Executive Officer of Weapons Company, 3d Battalion, 4th Marines, at the Marine Corps Air Ground Combat Center in Twenty-Nine Palms, CA. 1st Lieutenant Mercer exited active-duty service in July 2016.
In November 2018, Captain Mercer returned to active-duty operational support orders and was assigned to the Wounded Warrior Battalion East at Camp Lejeune, NC, as the Anti-Terrorism Force Protection & Assistant Operations Officer. He served as the lead planner for Marine Corps Mini-Trials and Team Officer-in-Charge for the Marine Corps Games.
Captain Mercer joined India Company, 3d Battalion, 25th Marines, in the summer of 2021 as a rifle platoon commander in Johnson City, TN. During that time, the company executed the Integrated Training Exercise at Twenty-Nine Palms, California.
In October 2022, Captain Mercer assumed the role of Executive Officer for Bravo Company, 1st Battalion, 24th Marines in Terre Haute, IN. The company successfully executed Infantry Officer's Course FINEX as the ADFOR in Fola, WV.
In September 2023, Captain Mercer assumed command of Charlie Company, 1st Battalion, 24th Marines. In February 2024, he mobilized and deployed with the 24th Marine Expeditionary Unit, serving as the Assistant Operations Officer for Battalion Landing Team 1st Battalion, 8th Marines. The deployment maintained a force posture in the Eastern Mediterranean for potential Non-Combatant Evacuation Operations and included bilateral training exercises such as BALTOPS 24, KSA 70, and Alexander the Great 24. Upon returning from deployment, Major Mercer resumed command of Charlie Company, 1st Battalion, 24th Marines in March 2025.
Major Mercer's personal awards include the Navy Commendation Medal (one gold star in lieu of a second award) and the Navy and Marine Corps Achievement Medal.