Colonel Mahar was raised outside of Albany, New York and enlisted in the United States Marine Corps Reserves in 1995 while attending East Stroudsburg University in Pennsylvania. Following recruit training at MCRD Parris Island, he served with Company F, 2d Battalion, 25th Marines until 1998.  Colonel Mahar commissioned in 1998 through the Platoon Leaders Course Program.

Colonel Mahar’s operational assignments include: Weapons Platoon Commander, 81mm Mortar Platoon Commander, and Weapons Company Executive Officer with 2d Battalion, 6th Marines (1999-2002).  During this time, he deployed with the 24th Marine Expeditionary Unit (Special Operations Capable) and then with the 22d MEU (SOC) in support of the Global War on Terrorism.  Colonel Mahar served with 1st Battalion, 6th Marines (2006-2008) and commanded Weapons Company through the battalion’s deployment to Ar Ramadi, Iraq and then was reassigned as operations officer through the battalion landing team’s deployment to Afghanistan with the 24th MEU.  Colonel Mahar commanded 3d Battalion, 6th Marines (2015-2017) through the battalion landing team's deployment with the 24th MEU and operations in support of Operation Inherent Resolve in Syria (2017).

Colonel Mahar’s service outside the Fleet Marine Force includes: Warfighting Instructor, Staff Platoon Commander, and Company Executive Officer at The Basic School (2002-2005), Training Officer for 2d Marine Regiment (2009), Commanding Officer Recruiting Station San Francisco (2009-2012), Joint Duty as a Military Aide to Vice President Joseph R. Biden Jr. (2013-2015), Current Operations Officer for 2d Marine Division (2018), and Director of Operations and Training for Expeditionary Warfare Training Group, Atlantic (2019-2022).

In addition to The Basic School and the Infantry Officers Course (1998-1999), Colonel Mahar is a graduate of Expeditionary Warfare School (2005), a distinguished graduate of Marine Corps Command and Staff College where he earned a Masters Degree in Military Studies (2013), and a graduate of The Joint Forces Staff College JCWS JPME-II Course (2021).  Colonel Mahar was also a National Security Affairs Fellow at the Hoover Institution at Stanford University (2019).

Colonel Mahar’s personal decorations include the Defense Superior Service Medal, Legion of Merit, Bronze Star Medal (with combat distinguishing device and two gold stars), Meritorious Service Medal, Navy Commendation Medal (with gold star), Combat Action Ribbon, and the Vice Presidential Service Badge.