GySgt Dempsey enlisted in the Marine Corps in September of 2002.  Upon completion of recruit training at MCRD Parris Island, South Carolina and Marine Combat Training at Camp Geiger, North Carolina, he reported to the Armed Forces School of Music in Norfolk, Virginia to attend the Basic Music Course. Upon graduation in July of 2003 he reported to Twentynine Palms, California for duty as a saxophone instrumentalist with the Combat Center Band. 
 
In August of 2006, GySgt Dempsey reported to Parris Island, South Carolina for duty with the Parris Island Marine Band. During this tour, SSgt Dempsey served as saxophone Section Leader, Library NCOIC, and MWR NCOIC.
 
In September 2009, GySgt Dempsey transferred to the III (3) MEF Band in Okinawa, Japan where he served as saxophone Section Leader, Administration Chief, and Assistant Drum Major.
 
GySgt Dempsey returned to the Naval School of Music in January of 2012 to attend the Unit Leader’s Course and upon graduation, reported to the 1st Marine Division Band in Camp Pendleton, California.  While serving with the Division Band, GySgt Dempsey served as saxophone Section Leader, Operations/Administration Chief, Acting Drum Major, and Guard Chief for the Division Main.
 
In December of 2015, GySgt Dempsey rotated to the Naval School of Music to serve as a Drill Band and Drum Major Techniques Instructor.  In addition to teaching responsibilities, GySgt Dempsey served as a Platoon Sergeant and Detachment Administrative Chief.
 
In July of 2018, GySgt Dempsey transitioned to Marine Band San Diego to serve as the Drum Major. In addition to his duties as the Drum Major, GySgt Dempsey served as a Watch Chief for the Depot Emergency Operations Center in response to COVID-19. Furthermore, GySgt Dempsey led and trained multiple small ensembles as the Small Ensemble Leader from August of 2021 until his departure in July of 2022.
 
GySgt Dempsey began his current tour as the Drum Major for the Quantico Marine Corps Band on 25 August, 2022.