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they formed, the American Professional Football Conference, was mostly an informal agreement to play a common schedule and name a champion at the end of each season of play. Teams were allowed to play games outside of the league, and membership was fluid in the early years. Two years later, the league renamed itself the National Football League. Only two teams currently in the NFL（NFL jerseys on sale）, the Decatur Staleys (now the Chicago Bears) and the Chicago Cardinals (now the Arizona Cardinals), are founding members.

League membership gradually stabilized throughout the 1920s and 1930s as the league adopted progressively more formal organization. The first official championship game was held in 1933. The NFL stopped signing black players in 1927 but reintegrated in 1946 following World War II. Other changes followed after the war; the office of league President evolved into the more powerful Commissioner post, mirroring a similar move in Major League Baseball. Teams became more financially viable, the last team folding in 1952. By 1958, when that season's NFL championship game became known as "The Greatest Game Ever Played", the NFL was on its way to becoming one of the most popular sports leagues in the United States.

The rival American Football League was founded in 1959. It was very successful, and forced a merger with the older NFL that resulted in a greatly expanded league and the creation of the Super Bowl, which has become the most-watched annual sporting event in the United States. The league continued to expand to its current size of 32 teams. A series of labor agreements during the 1990s and increasingly large television contracts has helped keep the league one of the most profitable and the only major league in the U.S. since 1990 to avoid a major work stoppage.