Sergeant Major Aaron G. McDonald entered the Marine Corps in June 1995 and graduated from recruit training in September of that year. Following graduation, then Lance Corporal McDonald attended his MOS school at the School of Infantry, Camp Pendleton, California. In December of 1995 Lance Corporal McDonald graduated as an Infantry Mortarman and was ordered to 1st Battalion, 3rd Marine Regiment in Kaneohe Bay, Hawaii. In September of 1998 Sergeant McDonald was transferred to Regimental Schools, 3rd Marine Regiment for duty as an instructor for the Squad Leader's Course.

In April of 1999 Sergeant McDonald was ordered to Drill Instructor Duty, Parris Island, South Carolina. Beginning in June of 1999, Sergeant McDonald served as a Drill Instructor and Senior Drill Instructor for 2nd and Support Recruit Training Battalions. In June of 2002 Staff Sergeant McDonald received orders to 3rd Battalion, 6th Marine Regiment in Camp Lejeune, North Carolina.

From June of 2002 to March of 2005 Staff Sergeant McDonald served as a Rifle and Weapons Platoon Sergeant. During his tenure Staff Sergeant McDonald was promoted to the rank of Gunnery Sergeant and received orders to the Staff Noncommissioned Officer's Academy, Camp Lejeune, North Carolina. While assigned to the Academy, Gunnery Sergeant McDonald served as an instructor and Staff Noncommissioned Officer in Charge of the Advanced and Sergeant's Courses.

Following his frocking to First Sergeant in January of 2007, First Sergeant McDonald was ordered 1st Battalion, 2nd Marine Regiment in Camp Lejeune, North Carolina. From January 2007 to December of 2008 First Sergeant McDonald served as the Weapons Company First Sergeant and in December of 2008 First Sergeant McDonald was assigned to I&I Staff, Company B, 6th Motor Transport Battalion, Lubbock, Texas. There he served until June of 2010 where he was reassigned to 1st Marine Special Operations Battalion, Camp Pendleton, California. While assigned to 1st Marine Special Operations Battalion, First Sergeant McDonald held billets as the H&S and B Company First Sergeant.

First Sergeant McDonald was frocked to the rank of Sergeant Major in July of 2012 and assigned to Recruiting Station Denver, 8th Marine Corps District, Denver, Colorado. From July 2012 to February 2015 Sergeant Major McDonald served as the Recruiting Station's Sergeant Major. In February of 2015 Sergeant Major McDonald was ordered to 2nd Assault Amphibian Battalion, 2nd Marine Division, Camp Lejeune, North Carolina for duty. Sergeant Major McDonald served as the Battalion Sergeant Major for 2nd Assault Amphibian Battalion until ordered to the Marine Raider Regiment, United States Marine Corps Forces Special Operations Command. There he served as the Raider Regiment's Sergeant Major until receiving orders to Joint Task Force North, United States Northern Command in November of 2017. Sergeant Major McDonald served as the JTF Command Senior Enlisted Leader conducting counter narcotic and United States border operations until he was assigned to United States Marine Corps Forces Europe and Africa. In July of 2020 Sergeant Major McDonald assumed the post of Command Senior Enlisted Leader, United States Marine Corps Forces Europe and Africa until June of 2022. In July of 2022 assumed the post of Command Senior Enlisted Leader, Education Command and Marine Corps University until July of 2023. In August of 2023 Sergeant Major McDonald assumed his current position as the Command Senior Enlisted Leader of Fleet Marine Forces Atlantic, Marine Forces Command, and Marine Forces North.

Sergeant Major McDonald is a graduate of Keystone 18-2 and Summit Class 8. He holds a BA in Management from American Military University and an MS in Strategic Leadership from the University of Charleston. His personal decorations include the Defense Superior Service Medal, Legion of Merit (2), Bronze Star Medal with Combat Distinguishing Device, Meritorious Service Medal (3), Joint Service Commendation Medal, Navy and Marine Corps Commendation Medal (5) with Combat Distinguishing Device, Navy and Marine Corps Achievement Medal (2), and the Combat Action Ribbon (2).