De Jesús debuted as a professional in 1969, beating El Tarita by a knockout in round three in San Juan. He won his first twenty fights, thirteen by knockout and then stepped up in class, for the first time, when he boxed future world title challenger Josue Marquez in 1971, beating him in a ten round decision. His next fight was a fourth round knockout victory over Victor Ortíz. After that, there was a rematch with Marquez, who was beaten again, this time over twelve rounds.