LtCol Hartnett commissioned in 2007 following graduation from the Illinois Institute of Technology. He received his Naval Aviator designation in 2010 and was selected to fly the AV-8B Harrier. After initial training in the Harrier at MCAS Cherry Point, Captain Hartnett reported to Marine Air Group 13 in Yuma, Arizona where he joined Marine Attack Squadron Five One Three (VMA-513) "Nightmares."

Captain Hartnett's time with VMA-513 was highlighted with exercises to include Mountain Roundup, Green Flag, and Enhanced Mojave Viper (EMV) II. In October 2012, Captain Hartnett was re-assigned to Marine Attack Squadron Two One One (VMA-211) "Avengers," in anticipation of the VMA-513 deactivation.   

Shortly after reporting to VMA-211, Captain Hartnett completed the Weapons Training Officer Course, Landing Signal Officer qualification, and graduated from the Aviation Safety Officer School. In 2014, he deployed with the 11th Marine Expeditionary Unit (MEU) aboard the USS Makin Island (LHD-8) in support of Operation ENDURING FREEDOM and Operation INHERENT RESOLVE. 

Upon return from deployment, Captain Hartnett reported to First Air Naval Gunfire Liaison Company (1st ANGLICO) on Camp Pendleton. During this tour, he earned the title of Honor Graduate for both the Tactical Air Control Party (TACP) School and Air Officer Division (AOD) Weapons and Tactics Instructor (WTI) course 2-16. He participated in multiple exercises including Emerald Warrior, EMV II, Japanese SACEX, Yuma ORDEX, MEFEX, Green Flag, and AC-130U integration.  

In 2016, the USMC Aviation Conversion Board selected Captain Hartnett as the USMC representative to F-35 Carrier Variant Initial Operational Test and Evaluation (IOT&E). He reported to Strike Fighter Squadron One Zero One (VFA-101) "Grim Reapers," on Eglin AFB for initial F-35C flight training. After completing training with VFA-101, Major Hartnett executed orders to Edwards AFB where he joined Air Test and Evaluation Squadron Nine (VX-9) Det Edwards “Vampires.” Major Hartnett served as the Operational Test Director until completion of IOT&E and then returned to the fleet where he joined Marine Fighter Attack Squadron Three One Four (VMFA-314) "Black Knights."  

Beginning in 2020, Major Hartnett served as the VMFA-314 Aviation Maintenance Officer while the squadron achieved Initial Operational Capable and then Full Operational Capable status as an F-35C squadron. Major Hartnett then moved to the Executive Officer position as the squadron deployed in 2022 with Carrier Air Wing Nine (CVW-9) aboard the USS Abraham Lincoln (CVN-72), marking the Marine Corps’ first F-35C deployment. Upon returning from deployment, LtCol Hartnett was selected for command of Marine Fighter Attack Squadron Five Three Three (VMFA-533) and temporarily reassigned to Marine Aircraft Group 11 (MAG-11) as the Headquarters Commanding Officer and Group Operations Officer until his move to MCAS Beaufort.  

LtCol Hartnett is a graduate of the Expeditionary Warfare School non-resident program and the Air Force Command and Staff Online Master’s Program. LtCol Hartnett has accumulated more than 1,700 flight hours in the AV-8B, F-35 and various other aircraft. He is the recipient of three Navy Commendation Medals.