When Rondón turned professional, his year of birth was listed as 1944; in fact it was 1938. Rondón began fighting as a middleweight and made a name for himself with upset victories over former world welterweight champion Luis Rodríguez and middleweight contender Bennie Briscoe. However, Rondon was growing and quickly established himself as a top rated light heavyweight with impressive wins over Roger Rouse, Eddie Talhami, and Allen Thomas. World Light heavyweight Champion Bob Foster did not seem eager to fight the #2 ranked Rondón or the #1 rated Jimmy Dupree. The WBA stripped Foster of his title and matched Rondón and Dupree.
In an exciting slugfest, Rondón climbed off the canvas in the second round to stop the favored Dupree at 2:58 of the 6th round. At the time of the stoppage, referee Zack Clayton had the fight scored even at 47–47. Judge Dimas Hernandez also scored the bout 47–47, and judge Gustavo Vargas favored Rondón, 48–47. Following the fight, a controversy broke out with Dupree claiming he was drugged. The United Press International published a story of the claim. In the UPI article, Charliese Smith, a registered nurse and friend of Dupree said, "I believe Jimmy was drugged. I saw Jimmy after the fight and he was very very weak. His vision was blurry and he couldn't even see the other side of the room." She went on to say, "I know of muscle relaxants that can be administered in food and I'm convinced that Jimmy was given something." Regardless of the charges, the World Boxing Association recognized Rondón as world champion, while The Ring magazine viewed Bob Foster as the legitimate Light heavyweight king.
1971 was an outstanding year for Rondón with a number of title defenses. He became the first and only fighter to stop Gomeo Brennan. Many experts felt that Rondón could and would defeat Bob Foster. Rondon flopped badly against Foster in 1972; being destroyed in two rounds. A venture into the heavyweight ranks proved just as bad, as he was beaten by Earnie Shavers, Ron Lyle and José Urtain. Rondón did better in 1973 as he started to trim down in weight. He looked highly skilled and impressive by scoring a 10 round decision over undefeated prospect Oliver Wright on Miami Beach. Shortly after he stopped heavyweight Mike "Jim" Boswell in four rounds.
Sadly, Rondón's private life was full of demons. He was developing a drinking problem and spending money like it was going out of style. An attempt to regain the light heavyweight title, saw him drop a decision to number 1 rated Len Hutchins, and then suffer a 9th round technical knockout to John Conteh. In 1974, Rondón, weighing 188 pounds, went back to the heavyweight division, where he would be largely unsuccessful. He was unable to get off the stool for round three in his fight with Rodney Bobick at Miami Beach, Florida. In his next fight he was knocked out in two rounds by former world heavyweight title contender José "King" Roman.