Mac McAllister was born on December 14, 1928, in Toledo, Ohio. He enlisted in the U.S. Marine Corps on March 18, 1946, and received an honorable discharge in May 1947. After completing two years of college, he entered the Aviation Cadet Program of the U.S. Air Force on September 23, 1949, and was commissioned a 2d Lt and awarded his pilot wings on October 28, 1950. Lt McAllister next completed F-86 Sabre Combat Crew Training at Nellis AFB, Nevada, in January 1951, and then served with the 16th Fighter Interceptor Squadron at Osan AB, South Korea, from February to November 1951. During this time he was credited with the destruction of 1 enemy aircraft in aerial combat, and he also flew as a forward air control pilot for 2 months in June and July 1951. His next assignment was as an F-86 pilot with the 94th Fighter Interceptor Squadron at George AFB, California, from November 1951 to April 1953, followed by service as an F-86 pilot with the 357th Fighter Interceptor Squadron at Portland International Airport in Portland, Oregon, and later at Nouasseur AB, Morocco, from April 1953 to February 1955. Capt McAllister then transferred to the 45th Fighter Day Squadron at Sidi Slimane AB, Morocco, where he served from February to October 1955. He served with the 1737th Ferry Squadron at Dover AFB, Deleware, from October 1955 to March 1958, followed by service as commander of the 4440th Aircraft Delivery Group at Kindley AFB, Bermuda. Maj McAllister deployed to Southeast Asia in December 1964, and served as an O-1 Bird Dog and O-2 Milirole forward air control pilot with the 21st Tactical Air Support Squadron at Qui Nhon AB, South Vietnam, until he was killed in action on April 22, 1965. William McAllister was buried at Fort Rosecrans National Cemetery in San Diego, California.

His Air Force Cross Citation reads:

The President of the United States of America, authorized by Title 10, Section 8742, United States Code, takes pride in presenting the Air Force Cross (Posthumously) to Major William Walter McAllister, United States Air Force (Reserve), for extraordinary heroism in military operations against an opposing armed force while serving as Pilot of an O-2 aircraft with the 21st Tactical Air Support Squadron, Qui Nhon Air Base, Vietnam, in action over the Republic of Vietnam on 9 and 10 March 1965. On these dates, acting as a Forward Air Controller under extremely adverse weather conditions and at night, he directed numerous air strikes in support of the Vietnamese Marines and their three American advisors. With complete disregard for his own safety, Major McAllister sought out targets ahead of the advancing Marines while under prolonged periods of hostile ground fire at dangerously low altitudes. Through his personal efforts, two American casualties were evacuated and the entire Marine unit effectively assisted in obtaining and securing their objectives. He also flew under extremely hazardous conditions under a low ceiling at night in mountainous terrain in an effort to assist friendly positions. Through his extraordinary heroism, superb airmanship, and aggressiveness in the face of hostile forces, Major McAllister reflected the highest credit upon himself and the United States Air Force.