After the war Garrison spent most of his career in special operations units, including the U.S. Army Intelligence Support Activity and the 1st Special Forces Operational Detachment-Delta (also known as Delta Force). His last command was the John F. Kennedy Special Warfare Center. He is known for being the commander in charge of Task Force Ranger during Operation Gothic Serpent in Somalia. Garrison retired from the Army on 3 August 1996, the day after Mohammed Farrah Aidid died, at the rank of Major General. Throughout his career, as he was promoted to Colonel, Brigadier General and Major General, Garrison was the youngest officer to hold those ranks.
Garrison is also said to have been in charge of the assistance force guiding the takedown of Pablo Escobar.
Garrison earned his Bachelor of Business Administration from Pan American University and graduated with a Master of Business Administration from Sam Houston State University.