Jackman attended the University of Nebraska-Lincoln on an Army ROTC scholarship. While at the University he became an active member of the National Society of Pershing Rifles, an ROTC military fraternal organization known for including many who later became successful Army officers. Upon graduation he was commissioned into active duty in the U.S. Army in June 1973. His first duty assignment was with the 1st Battalion (ABN) 508th Infantry, 82nd Airborne Division, Fort Bragg, North Carolina, where he served as a rifle platoon leader, anti-tank platoon leader, company executive officer, and battalion adjutant. He next served with the 1st Battalion, 21st Infantry, 25th Infantry Division, Schofield Barracks, Hawaii, as a battalion S4, rifle company commander, and battalion S3. Following assignment as a Procurement Officer with the United States Missile Command, Redstone Arsenal, Alabama, he served at Fort Bragg as Procurement Officer, Squadron Executive Officer, and Support Squadron Commander in 1st Special Forces Operational Detachment-Delta (Airborne) (1st SFOD-D(A)), more commonly known as "Delta Force."