Why did Cassius Clay (Muhammad Ali) become a boxer? As a 12-year-old boy, he was given a beautiful new bicycle as a gift. The bicycle was stolen, and when Cassius went to the local police department to report the theft, he met officer Joe Martin. Martin introduced Cassius to the world of boxing, and this was the beginning of his boxing career. Also Cassius vowed that he was "going to whip whoever stole his bike," but the stolen bicycle never turned up.