Ford is actually from a small planet somewhere in the vicinity of Betelgeuse, and had originally only come to Earth to do some research for an article he was writing on it for the Guide.

Ford came to Earth for a week, and got stuck there for fifteen years, departing only when a fleet of Vogon constructor ships appear in the first episode, taking Arthur Dent with him. Ford is the source of much explanation of the weird universe that Arthur finds himself in; for example, the importance of knowing where your towel is, sticking a fish in your ear, and why the greatest cooks in the universe cook on Vogon spaceships. Ultimately, however, his personal mission is to find a nice party and get incredibly drunk.