In 1982, Mackmull led the committee that determined the army's future helicopter requirements, including an air-to-air combat capability.

[5] While at Roosevelt, Mackmull joined the National Honor Society, played football, basketball, and baseball, and was considered one of Ohio's most prominent high school athletes.

[14] After receiving his commission, Mackmull was assigned to Company G, 4th Regimental Combat Team (RCT), a unit of the 2nd Infantry Division then based at Fort Lewis, Washington.

[21] After completing his fixed-wing aviation training, in 1960 Mackmull was promoted to major and selected for attendance at the United States Army Command and General Staff College.

[6] In 1964, Mackmull was promoted to lieutenant colonel and assigned to duty in South Vietnam as commander of the 13th Combat Aviation Battalion.

[6] After organizing the battalion and leading it overseas, he was replaced and assigned to temporary duty as a staff officer with the Army Concept Team in Vietnam, for which he documented the use of helicopters in the war and made recommendations for equipment procurement and fielding, as well as personnel training.

[6] In December 1968, Mackmull was again assigned to Vietnam, this time as commander of the 164th Combat Aviation Group at Cần Thơ Base Camp.

[12] In this role, he oversaw the army's efforts to improve automated systems that accounted for unit personnel, equipment, vehicles and weapons.

[12] In June 1977, Mackmull was assigned as commander of the John F. Kennedy Special Warfare Center and School, and he was promoted to major general in July.

[28] As head of Special Forces, Mackmull led post-Vietnam War modernization of equipment, doctrine, tactics, and command relationships.

[6][12] The 160th Aviation Regiment supports Special Operations by helicopter attack, assault, and reconnaissance, frequently on short notice, at night, and at high speed and low altitude.

[6] The committee's report and recommendations led to recognition of a future requirement for a helicopter that could engage in air-to-air combat and the 1983 establishment of the Army Aviation Branch.

[6] In addition, Mackmull oversaw the training for, planning, and execution of the army's participation in the United States invasion of Grenada.

[6][33] For several years, he provided advice and guidance on Special Operations and aviation to the army as president of the Mackmull Associates consulting firm.

[6] After moving to Melbourne, Mackmull resided at the Indian River Colony Club (IRCC) retirement community.