López caught the attention of scout Ramon "La Chita" García of the Mexico City Reds in the Mexican League. He debuted with the team's minor-league affiliate in Las Choapas in 1967. He was promoted to the parent club the next season at the age of 19. After starting the 1969 season with Mexico City, López joined the Minatitlán Red Devils. In 1970, López rejoined Mexico City. During his time in the Mexican League, he was converted from starter to reliever by coach Wilfredo Calviño.