It is difficult to determine when pro football began. It all depends on your definition of "professional". Ringers were abundant among teams. In 1888, the Amateur Athletic Union (became the National Collegiate Athletic Association in 1905) was formed to combat this and to keep amateur athletes as just that: amateurs.
The first player to have openly admitted to accepting payment for his services is John K. Brallier in 1895. However, pro football researchers claim that Walter "Pudge" Heffelfinger was the first football player to be paid for his services. Researchers claim that in 1892, the Allegheny Athletic Association and the Pittsburgh Athletic Club were competing for Heffelfinger's services (Allegheny Athletic Association being the final recipient of his services). Heffelfinger never acknowledged payment for his services.
Athletic clubs were prevalent in western Pennsylvania. Formal competition began in 1890. The two main players were the Pittsburgh Athletic Club (known as the East End Gymnasium Club until 1892) and the Allegheny Athletic Association, however, they did not play each other until October 1892. The result was a 6-6 tie. Allegations of unsportsmanlike conduct and the use of ringers increased the intensity of this rivalry. They met again November 12. The Allegheny Athletic Association won this 4-0. Truly professional teams started in western Pennsylvania in 1896 with the Allegheny Athletic Association. The Latrobe Athletic Club and the Greenburg Athletic Association began in 1897.
Interestingly enough, baseball had an effect on the formation of professional football. The National League's Philadelphia Phillies and the American League's Philadelphia Athletics had a natural rivalry. This rivalry would extend into more than baseball. In 1902, Phillies owner John I. Rogers formed a football team. Athletics owner Ben Shibe then formed a team to compete (and beat) Rogers' team. Both teams contacted Dave Berry (former manager of the Latrobe Athletic Club) to form a team. These three teams formed the National Football League, the first professional football league. This league did not last long. As soon as 1903, players were leaving the NFL for pro teams forming in Ohio.
The "Ohio League" was loosely formed of professional teams in Ohio. The Canton Bulldogs and the Massillon Tigers were perennial powerhouses. The league, in large part, contributed to the development of professional football; but by 1919 the league was having trouble. Teams outside Ohio were signing away the state's best players. They were also having trouble getting enough gate receipts to cover the players salaries. With the average team salary around $2000 and each paying customer admitted for only one dollar, it was easy to see how the teams got themselves into financial troubles. This was when people started thinking about a national professional football league.