Garrett was commissioned in the United States Army as an infantry officer after graduating from North Georgia College in Dahlonega, Georgia in 1981. He graduated from the Infantry Officer Basic and Advanced Courses, the United States Army Command and General Staff College, the School of Advanced Military Studies, and the National War College. He holds a Master of Arts degree in management from Webster University, a Master of Arts degree in Military Art and Science from the U.S. Army Command and General Staff College, and a Master of Science degree in National Security and Strategic Studies from National Defense University.^{[citation needed]}

Garrett became the Deputy Commander of the United States European Command on June 10, 2010. Before that, he served as Deputy Commanding General and Chief of Staff for United States Army Forces Command on December 31, 2012, in Fort Bragg. He was responsible for manning, equipping, and training 265,000 active component soldiers, and training and readiness oversight of 560,000 soldiers of the Army National Guard and the United States Army Reserve. Garrett also served as Chief of Staff, United States Forces Iraq, and as Commanding General, United States Army Africa and United States Army Southern European Task Force, Vicenza, Italy.

Other assignments include tours of duty with the 6th Infantry Division; 10th Mountain Division; 25th Infantry Division; 82nd Airborne Division; XVIII Airborne Corps; United States Army Combined Arms Center; the United States Special Operations Command; the Army Staff; and the Joint Staff. He has served operational tours in Grenada, Egypt, Syria, Afghanistan. He served on the board of advisors of the Military Cyber Professionals Association. And currently on assignment with the UN peacekeeping mission since June 2023 in Iraq^[2]

LTG Garrett has received the following awards:^[3]

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