He met boxing trainer Carmelo Prada, who helped shape his style. Cervantes only had three amateur bouts, winning two and losing one. On January 31, 1964, Cervantes entered the boxing ring as a professional for the first time, beating Juan Martinez by decision in six rounds. His first 32 bouts as a professional were in Colombia, and he won 27, lost 4 and drew (tied) one of them. Realizing his pupil needed publicity outside Colombia, Prada then moved with Cervantes to Venezuela, where , on November 25, 1968, he knocked out Orlando Ruiz in the first round for his first fight abroad. He followed that win with a ten rounds decision over Nestor Rojas in the very short time of only three days later.
On December 23 of that year, he suffered his first knockout defeat when Cruz Marcano, a fringe contender of the time, beat him in four rounds in Caracas.
He won five and lost two in 1969, splitting his fights between Colombia and Venezuela, and losing a ten round decision to Antonio Gomez, former world champion.
He won two more in 1970, then he and Prada moved once again, this time to Los Angeles. In California, he began by beating Jose Rodriguez by a knockout in the first round in San Jose, and on December 17, he beat Rodolfo Gato Gonzalez, a very famous Mexican boxer of the time, by knockout in round eight. After this and a 1971 win over Argentine Enrique Jana, Cervantes was ranked as a Jr. Welterweight by the WBA. On December 11 of that year, he had his first world title try, against Nicolino Loche, in Buenos Aires. Cervantes lost a 15 round decision that time, however.
After winning three fights in 1972, Cervantes, who had by then returned to Colombia, had another world title try: Alfonso Peppermint Frazer, who had dethroned Loche of the world title, gave Cervantes a chance on October 28 at Panama City. Cervantes knocked Frazer out in round ten and became world champion for the first time.
Cervantes immediately became a national hero in Colombia, and many enterprises made him their spokesman in the coffee producing country, most notably Sanyo, whose name the popular champion advertised on his clothing and fight trunks (boxer's shorts).
Cervantes made nine defenses, including a knockout in nine of Loche in a rematch, a knockout in five of Frazer, also in a rematch, a 13th. round knockout win over Josue Marquez in Puerto Rico and a 15 round decision against future world Lightweight champion Esteban De Jesus. But on March 6, 1976 at San Juan, he lost a 15 round decision and the world title to 17 year old Puerto Rican Wilfredo Benitez, who with that became boxing's youngest champion ever.
Cervantes won five more fights in a row, including a win over Saoul Mamby, before fighting for the world title again. After Benitez had left the Jr. welterweight belt vacant, Cervantes regained it on June 25, 1977, with a five round knockout over Carlos Maria Gimenez, once again in Venezuela. His second reign as world champion took him to such places as Thailand, Botswana and South Korea, among others. He retained the title five times, beating the likes of Adrian Marrero and Miguel Montilla (twice), among others. By this time, there was much talk about a superfight with world Lightweight champion Roberto Duran, who was coming up in weight. Duran decided to challenge Sugar Ray Leonard instead, however, and Cervantes vs. Duran never materialized.
On August 2, 1980, Cervantes dropped his next title challenger, Aaron Pryor, to the canvas in the first round. Pryor recovered, however, and beat Cervantes by a knockout in round four. This turned out to be Cervantes' last title fight.
Cervantes went on boxing until 1983, winning four fights and dropping his last one, a ten round decision loss to Danny Sanchez on December 9, 1983 at Miami.
In retirement, his life was almost as public as it was during his boxing career: In 1985, for example, he and a passenger in one of the boats Cervantes owned, suffered a water accident, and the passenger almost drowned, but Cervantes was able to save her life.
Cervantes was inducted into the International Boxing Hall of Fame in 1998, and in 2000, he was declared by the Colombian Boxing Federation and National Association of Professional Boxing as Colombia's Fighter of the Century. For that award, the WBA issued him a special, commemorative belt. Cervantes' nickname, Kid Pambele, became almost as famous as Cervantes himself: as a matter of a fact, many fans called him only Kid Pambele.
His record has been of some dispute. Listed at 91 wins, 12 losses and 3 draws with 45 knockouts, some websites and fans allege that he actually won 66, with 12 losses, only 1 draw and 37 knockouts.