Monsoor enlisted in the United States Navy on 21 March 2001, and attended Basic Training at Recruit Training Command Great Lakes, Illinois. Upon graduation from basic training, he attended Quartermaster "A" School, and then transferred to Naval Air Station Sigonella, Italy for a short period of time. He entered Basic Underwater Demolition/SEAL (BUD/S) training and graduated from Class 250 on 2 September 2004, as one of the top performers in his class. After BUD/S, he completed advanced SEAL training courses including parachute training at Basic Airborne School, cold weather combat training in Kodiak, Alaska, and six months of SEAL Qualification Training in Coronado, California, graduating in March 2005. The following month, his rating changed from Quartermaster to Master-at-Arms, and he was assigned to Delta Platoon, SEAL Team 3.