De Leon, a native of Trujillo Alto, Puerto Rico, first won a world title when faced with WBC world champion Marvin Camel on November 25 1980, at the undercard of Sugar Ray Leonard and Roberto Duran's second fight in New Orleans. De Leon outpointed Camel over 15 rounds. After he knocked out Camel in 8 in a rematch, countryman Ossie Ocasio won the WBA world title, becoming the second pair of Puerto Ricans to share world titles in the same division at the same time, after Alfredo Escalera and Samuel Serrano had achieved the feat in the 1970s at Jr. Lightweight. De Leon lost his title in a shocking upset to former Gerry Cooney victim St. Gordon by a knockout in 2 at Cleveland in 1982, and won a comeback fight versus former world Heavyweight champion Leon Spinks by a knockout in six in 1983. After that, he and Gordon boxed a rematch in Las Vegas, and De Leon dropped Gordon once in the first round and once in the twelfth, en route to a unanimous decision win in a history making bout: De Leon had now become the first boxer to win the world Cruiserweight title twice.
He defended his title against Alvaro Yaqui Lopez by a knockout in four at San Jose, California, and with decisions over Anthony Davis, Jose Maria Flores Burlon, and Bashiru Ali. The Davis and Burlon bouts took place in Las Vegas and the fight with Ali was in Oakland, California. De Leon next lost his title in Las Vegas to Alfonzo Ratliff by a decision. Ratliff was in turn beaten by Bernard Benton, who defended against De Leon on March 22 of 1986, once again in Las Vegas. De Leon joined the likes of Sugar Ray Robinson and Muhammad Ali in becoming one of the few boxers ever to win one division's world championship at least three times, defeating Benton by decision. He made a couple of defenses in Italy and then in 1988, lost his title in an unification bout with WBA world champion Evander Holyfield, by a knockout in eight, also in Las Vegas.
But Holyfield soon left the division to pursue the world Heavyweight championship, and De Leon was left with an open door to break his own record and win the title for a record fourth time. He went to London, where he beat the WBC's number 2 challenger, Sammy Steward, by a knockout in the fourth round, breaking his own record and crowning himself world Cruiserweight champion once again. This time, he held on to the title for two years, until losing it to Massimiliano Duran in Italy by an 11 round disqualification.
During the 1990s, De Leon ran afoul of the law a couple of times, once being found carrying a rifle on him. But nowadays, he is concentrated on helping the career of his son Carlos De Leon Jr