Reacher served 13 years in the Army, plus 4 years at West Point, from 1983 to 1997. He specifically served in the Military Police as a 2nd Lieutenant, Lieutenant, Captain, and Major. He was demoted from Major to Captain in the novel The Enemy, but was able to work his way back to Major before he mustered out.
Throughout his 13 years of service, Reacher won the Bronze Star, the Defense Superior Service Medal, the Legion of Merit, a Soldier's Medal, and a Silver Star and Purple Heart for wounds and actions in the bombing of the US Marine Corps barracks in Beirut in 1983. His Silver Star and Purple Heart are cited on his profile, but all of the other medal citations are redacted meaning they involved secrets.
He had many formal qualifications. He was rated expert on all small arms and is the only non-Marine to win the Wimbledon Cup, a US Marine Corps 1000 Yard Invitational Rifle Competition; he completed the competition with a record score in 1988. Anecdotally his fitness reports rated him well above average in the classroom, excellent in the field, fluently bilingual in English and French, passable in Spanish, outstanding on all man-portable weaponry, and beyond outstanding at hand-to-hand combat (his fighting style has been described as that of a running chainsaw being thrown).