Colonel Fennell graduated Officer Candidate School and commissioned as a 2nd Lieutenant in 1997. Under a law contract, he completed his law and master’s degrees before graduating from The Basic School and Naval Justice School w/ honors in 2001. From 2002 to 2004, he was a Judge Advocate (JAG) at Marine Corps Base Hawaii as Officer in Charge of Legal Assistance and a Military Justice federal prosecutor.
In 2006, to serve in the War on Terror, Captain Fennell entered the Selected Marine Corps Reserve and joined Marine Air Control Squadron 23 (MACS-23) for its future deployment to Iraq. In 2007, he left civilian employment to complete the Air Defense Control Officer Course before deploying to Al Asad, Iraq in 2008 as Detachment Commander, Tactical Air Operations Center (TAOC). He presented air command and control briefs for MACG-38 at U.S. Air Force Central Command in Qatar and for 3D MAW at the Weapons and Tactics conference in Bahrain. Afterward, he was assigned as Executive Officer, MACS-23.
In 2009, Major Fennell joined 4th Civil Affairs Group (CAG) to lead a civil affairs team in Helmand Province, Afghanistan. His team supported 3rd Battalion 10th Marines at Firebase Fiddlers Green in the shaping of Marjah before being re-assigned to 1st Battalion 6th Marines for Operation Moshturak. On February 13, 2010, Major Fennell led his team in the initial nighttime helicopter assault of central Marjah. He was later invited to brief international senior leaders at the Afghanistan Counter Insurgency Academy in Kabul, Afghanistan. In September 2010, Major Fennell returned to MACS-23 as the TAOC Detachment Commander where he supported Exercise Northern Edge at Eielson Air Force Base, Alaska. In 2011, he was Executive Officer, MACS-23 until the unit’s deactivation in September 2012. Combat Logistics Battalion 453 was activated in its place and Major Fennell was asked to remain as Executive Officer, CLB-453.
From 2013 to 2015, Lieutenant Colonel Fennell served as an Operations Watch Officer with U.S. Strategic Command, Joint Functional Component Command for Integrated Missile Defense (JFCC-IMD) in Colorado Springs, CO. In 2015, he was certified in MAGTF Information Operations and joined Marine Forces Pacific as the Information Operations Staff Officer where he supported U.S. Pacific Command exercises Pacific Sentry and Cobra Gold, Thailand. He commanded Marine Tactical Air Command Squadron-48 at Naval Station Great Lakes, IL from April 2017 to May 2019. He then attended the Naval War College in Newport, RI and was selected as an Arleigh Burke Fellow at the National Command College until graduating w/ distinction in 2020.
In August 2020, Colonel Fennell joined 3D MAW at Miramar, CA as Reserve AC/S G-3, supporting exercises Pacific Sentry and Summer Fury. From March 2022 to May 2024, he commanded 1st CAG and advised I Marine Expeditionary Force (I MEF) as the G-9 at Camp Pendleton, CA from March 2022 to May 2024 and led the Combined Joint Civil Military Operations Task Force (CJCMOTF) during Balikatan, Philippines. In June 2024, he was assigned as Officer in Charge of the new Force Headquarters Group Information Detachment in New Orleans, LA augmenting information warfare capabilities in support of Marine and Joint activities across the globe and transitioning to a reserve structured Marine Information Group. In November 2025, he joined Reserve Affairs, Career Management Team prior to being activated as Chief of Staff, 4th Marine Aircraft Wing in April 2026.
B.S. degree from Colorado State University (College of Veterinary Medicine & Biomedical Sciences Dean’s List), M.S.E.L degree magna cum laude from Vermont Law School, M.A. degree Top 10 w/ distinction from Naval War College, and J.D. degree from Lewis and Clark Law School. Graduate from Marine Corps University Command and Staff College and Reserve Senior Staff Course. Certificates in Senior Leadership. Development from University of North Carolina Kenan-Flagler Business School and Project/Program Management Level II from Federal Acquisition Institute.