20th century

Starting in 1925, the Pennsylvania National Guard used the present airport site (known as Hog Island) as a training airfield. The site was dedicated as the "Philadelphia Municipal Airport" by Charles Lindbergh in 1927, but it had no proper terminal building until 1940; airlines used Camden Central Airport in nearby Pennsauken Township, New Jersey. Once Philadelphia's terminal was completed (on the east side of the field) American, Eastern, TWA, and United moved their operations here.

In 1947 and 1950, the airport had runways 4, 9, 12 and 17, all 5,400 feet (1,600 m) or less. In 1956 runway 9 was 7,284 feet (2,220 m); in 1959 it was 9,499 feet (2,895 m) and runway 12 was closed. Not much changed until the early 1970s, when runway 4 was closed and 9R opened with 10,500 feet (3,200 m).

On June 20, 1940, the airport's weather station became the official point for Philadelphia weather observations and records by the National Weather Service.

During World War II the United States Army Air Forces used the airport as a First Air Force training airfield.

Beginning in 1940, Rising Sun School of Aeronautics of Coatesville performed primary flight training at the airport under contract to the Air Corps. After the Pearl Harbor Attack, the I Fighter Command Philadelphia Fighter Wing provided air defense of the Philadelphia metropolitan area from the airport. Throughout the war, various fighter and bomber groups were organized and trained at Philadelphia airport and assigned to the Philadelphia Fighter Wing before being sent to advanced training airfields or being deployed overseas. Known units assigned were the 33d, 58th, 355th and 358th Fighter Groups.

In June 1943, I Fighter Command transferred jurisdiction of the airport to the Air Technical Service Command (ATSC). ATSC established a sub-depot of the Middletown Air Depot at the airport. The 855th Army Air Forces Specialized Depot unit repaired and overhauled aircraft and returned them to active service, and the Army Air Forces Training Command established the Philco Training School on January 1, 1943, which trained personnel in radio repair and operations.

In 1945, the Air Force reduced its use of the airport and it was returned to civil control that September.

Philadelphia Municipal became Philadelphia International Airport in 1945, when American Overseas Airlines began direct flights to Europe. A new terminal opened in December 1953; the oldest parts of the present terminal complex (B and C) were built in the late 1950s.

As of April 1957, the airport was providing 30 weekday departures on Eastern, 24 on TWA, 24 on United, 18 on American, 16 on National, 14 on Capital, six on Allegheny, and three on Delta. To Europe, five Pan Am DC-6Bs a week via Idlewild and Boston and two TWA 749As a week via Idlewild; one TWA flight continued to Ceylon. Eastern and National had nonstops to Miami, but the TWA 1049G to LAX that started in 1956 was the only nonstop beyond Chicago. The first scheduled jets were TWA 707s in the summer of 1959.

Terminal B/C modernization was completed in 1970, Terminal D opened in 1973 and Terminal E in 1977; the $300 million expansion was designed by Arnold Thompson Associates, Inc. and Vincent G. Kling & Associates.

In the 1980s, the airport hosted several hubs. The Airline Deregulation Act of 1978 allowed regional carrier Altair Airlines to create a small hub at PHL using Fokker F-28s. Altair began in 1967 with flights to cities such as Rochester, New York, Hartford, Connecticut and to Florida until it ceased operations in November 1982. In the mid-1980s Eastern Air Lines opened a hub in Concourse C. The airline declined in the late 1980s and sold aircraft and gate leases to Chicago-based Midway Airlines. Midway operated its Philadelphia hub until it ceased operation in 1991. During the 1980s US Airways (then called USAir) built a hub at PHL.

US Airways became the dominant carrier at the airport in the 1980s and 1990s and shifted most of its hub operations from Pittsburgh to Philadelphia in 2003. As of 2013, the airport was US Airways' largest international hub and its second-largest hub overall behind Charlotte. PHL became an American Airlines hub after it completed its merger with US Airways in 2015 and remains one of the airline's biggest hubs, offering an average of 420 departing flights per day to over 100 destinations. In recent years, American has opted to continue expanding at PHL while downsizing its hub at JFK in New York due to greater slot availability, lower operation costs in Philadelphia, and its greater network of connecting flights.

In July 1999, the Pennsylvania Department of Transportation (PennDOT) and several U.S. federal government agencies selected a route for the connecting ramps from Interstate 95 to the Terminal A-West complex, then under development; the agency tried to avoid the John Heinz National Wildlife Refuge at Tinicum. K/B Fund II, the owner of the International Plaza complex, formerly the Scott Paper headquarters Scott Plaza, objected to the proposed routing, saying it would interfere with International Plaza development. It entered a filing in the U.S.