Lackland Air Force Base dates from July 4, 1942, when the War Department separated the part of Kelly Field lying west of Leon Creek and made it an independent installation, naming it the San Antonio Aviation Cadet Center. Prior to 1941, the portion of Kelly Field beyond Leon Creek known as "the Hill" served at various times as a bivouac area and bombing range for advanced aviation cadets going back to 1922. Construction of an Air Corps replacement-training center on the Hill began on 15 June 1941. The first class of 1,906 cadets began training in mid-November 1941, even though barracks were not ready until mid-December. The demand for aircrew became urgent after Pearl Harbor and inductees picked as aviation cadets began to pour into Kelly Field. On April 30, 1942, the War Department redesignated the replacement-training center as a preflight school and established a classification center. This expansion in classification operations caused a need for additional facilities, the construction of which began in June 1942.

Managing this explosion of activity on the Hill became increasingly burdensome for the commander of Kelly's advanced flying school, who focused on the urgent demand for bomber pilots. The Adjutant General took action on June 26, 1942, and directed that the Hill be separated from Kelly Field and operated as an independent military installation designated as the San Antonio Aviation Cadet Center. The Gulf Coast Army Air Forces Training Center, an intermediate higher headquarters, formally transferred the preflight school, the classification center, a station hospital, an Air Force band, and several other units to the San Antonio Aviation Cadet Center, activated on July 4, 1942.

With general mobilization following Pearl Harbor, the San Antonio Aviation Cadet Center grew rapidly. Approximately 90,000 candidates for flying training passed through the preflight school. On 5 November 1942, the SAACC Recruit Detachment received the first raw recruits for enlisted basic military training to fill the ranks of the Gulf Coast Training Command. Basic military training occurred in a tent city on the site of present day Wilford Hall Medical Center.

The San Antonio Aviation Cadet Center performed a variety of other training missions late in the war; among them were officer candidate training for enlisted men, indoctrination for officers directly commissioned, and preparation for officers to pursue advanced college courses. The Center closed on June 30, 1945, and the next day the Army redesignated the installation as the San Antonio District, Army Air Forces Personnel Distribution Command. The new mission involved receiving soldiers from the overseas theaters and reassigning or separating them. With that task largely accomplished by early 1946, the base became subject to the general demobilization.

Instead of closing operations on the Hill, as happened to most wartime training camps, the Army redesignated it as the Army Air Forces Military Training Center, on February 1, 1946, and gave it the basic military training mission for the Army Air Force. Reinforcing the military training center's indoctrination mission, the Officer Candidate School also transferred there from Maxwell Field, Alabama. On November 1, 1946, Air Training Command redesignated the military training center as the Indoctrination Division. While all the name changes reflected mission changes, the base still lacked an identity. The War Department finally resolved the identity crisis on July 11, 1947, by naming the base for Brigadier General Frank D. Lackland. General Lackland originated the idea of an aviation cadet reception and training center at Kelly.
Gateway to the Air Force "The Gateway to the Air Force" accurately described Lackland after 1946. Much of the time, it had sole responsibility for the military indoctrination of basic trainees. Only during the Berlin Airlift crisis and the Korean and Vietnam Wars did the Air Force find it necessary to conduct basic military training elsewhere. During the Korean War, Sheppard AFB, Texas, Parks AFB, California, and Sampson AFB, New York, also provided recruit training until shortly after the war ended. Crowded conditions and a meningitis outbreak caused Lackland to shift some basic trainees to Amarillo AFB, Texas, between February 17, 1966, and December 11, 1968. With those exceptions, after February 1946 all enlisted airmen began their Air Force careers at Lackland. Over the same period, Lackland had a large role in training future officers. Officer Candidate School produced reserve officers from the enlisted corps until July 1962; the Officer Training School activated on July 1, 1959, and commissioned college graduates with no prior service, as well as airmen who had earned undergraduate degrees. In 1993 the Officer Training School, moved to Maxwell AFB, Alabama.
Although Lackland's technical training mission grew out of the Korean War, Lackland gained its first permanent technical training responsibility in May 1954, when the Officer Candidate School implemented the USAF Recruiting Course. The base received its first technical training unit on November 20, 1956, when the 3275th Technical Training Group transferred from Parks AFB, California, bringing with it all air police training. In July 1957, cryptographic equipment maintenance training transferred from Scott AFB, Illinois. In 1958, the base gained two new missions connected with air police training: the USAF Marksmanship Center and a sentry dog handler course.

Teaching English to military personnel from foreign countries became another critical mission on Lackland. Begun informally in 1953, the Air Force established a formal English training program in May 1954, which gave way to the USAF Language School, which activated on January 1, 1960. The Department of Defense took over the mission in July 1966, and gave it to the Defense Language Institute, under the executive agency of the US Army. In October 1976, the Air Force became the executive agent for the Defense Language Institute English Language Center.

Despite the basic and technical training missions, Lackland's infrastructure still relied on temporary buildings dating to World War II. Facilities erected initially in 1941, and in two mobilizations (1942-1943 and 1951) dominated the Lackland landscape. Most of the approximately 1,400 buildings erected for World War II were in place when the Korean War began in June 1950. Korean War manpower needs greatly exceeded Lackland's capacity, even with other bases joining in the basic military training effort and resulted in another mobilization building project to increase the base's trainee/student capacity.
In 1957 Lackland got its first visible sign of permanence. A new nine-story, 500-bed hospital dominated the north rim of the base, displacing most of the 94 temporary buildings that had made up the hospital complex. The hospital added a 500-bed wing in 1961. On the east side of the base in 1971, the main Base Exchange complex took the place of 109 World War II barracks torn down between 1966 and 1971. At the same time, on the west side of Lackland, contractors built more facilities for recruit housing and training. Removing the old, temporary structures as rapidly as possible became a priority in 1976 and construction in the 1980s mostly focused on new unaccompanied permanent personnel quarters and technical trainee dormitories. Other projects tackled crumbling infrastructure: sewers, sewage treatment, water mains, and roads.
Wilford Hall
With the closure of Kelly AFB on 1 April 2001, the Air Force transferred the airfield operations mission and real property west of Kelly's hanger line Lackland. For the first time in its history, Lackland gained a runway, but training remained its most visible mission.