Enlisting in the Marine Corps Reserve at Phoenix, Arizona on June 7, 1968, Jiménez was discharged to enlist in the regular Marine Corps, August 12, 1968. He completed recruit training with the 1st Recruit Training Battalion at Marine Corps Recruit Depot San Diego, California, in October 1968. He was promoted to private first class on October 1, 1968. Transferred to the Marine Corps Base Camp Pendleton, California, he underwent individual combat training with Company G, 1st Battalion, 2nd Infantry Training Regiment and with the Rifle Training Company of the 2nd Infantry Training Regiment, completing the latter in December 1968.
