The NFL as we know it has really only been around since the 1960s. But professional football dates back far longer than that. The first noted professional football event took place in 1892. American football as a sport had existed before that, but a game between the Allegheny Athletic Association and the Pittsburgh Athletic Club in Pennsylvania in 1892 was the first instance of a player getting paid to play.
In the years following the first professional player, there were several attempts to get an official professional football league off the ground. In 1902 there was even one called the National Football League, ironically with teams of Philadelphia baseball players. But there wasn't much money or fanfare, and the league was soon gone.