The tradition of American Presidents throwing out the first pitch began with William Howard Taft, our nations 27th President. Shortly after his inauguration Taft was offered season tickets by Washington Senator Owner Thomas Noyes, and a few months later he was at his first game as President.
Having loved the experience Taft was there one year later on opening day to begin a long-standing tradition. Every President since Taft has thrown out an opening day pitch except for Jimmy Carter, although he did throw out the first ball in game 7 of the 79 World Series.
Over the years the tradition had evolved from Presidents throwing the ball from the stands, to having a gaggle of players fight over the Presidents toss. Ronald Reagan was the first President to actually throw from the field and Bill Clinton is credited as the first to make it to home plate on the fly.
Out of all the major sports, baseball is the only one in which the commander-in-chief has become an actual part of, truly emphasizing its place as Americas past time.