Casamayor won during his long amateur career 333 wins and 30 defeats. In 1989, he was in Bayamón junior world champion in the bantamweight class. In 1992, he took to Cuba in the Olympic Games in Barcelona and won the gold medal. In the amateur world championship 1993 in Tampere he finished second, but was thwarted in 1995 in Berlin as early as the first round on Russians Raimkul Malachbekow.
Casamayor was also provided in 1996 as a Cuban representative for the Olympic Games in Atlanta, sat down, however, before the tournament of the Cuban team and became a professional in the U.S..
Successes as an amateur
- Junior World Champion 1989
- Central American and Caribbean Games - Second 1990
- Olympic champion in 1992
- Chemistry Cup - Winner 1992, 1996
- Cuban champion in 1993
- Amateur World Championship - Second 1993
- World Cup - Third 1994
- Goodwill Games - Second 1994
His first professional fight denied Casamayor in September 1996 in Miami and won against the Americans David Chamendis by first-round KO. On 19 June 1999, he won by one point victory over Mexican Antonio Hernández the interim WBA title in the half- light weight, making him eligible to challenge the reigning WBA titlist. On May 21, 2000, there came to the encounter with the defending champion, the undefeated Korean Baek Jong- kwon. Casamayor won the fight and the title by technical knockout in the fifth round after the fight was stopped in his favor due to lacerations of Korean.
The WBA title he defended in the following period four times before it came to a unification fight with WBO titleholder Acelino Freitas on January 12, 2002. After a precipitate in the third round and a point deduction in the sixth round due lookup Casamayor lost the battle and the WBA title over twelve rounds just on points.
In January 2003, he defeated the unbeaten American Nate Campbell ( 23-0 ) and met on 4 October 2003 in an eliminator of the IBF to Diego Corrales. Both boxers had in the course of combat on the ground until the fight in the sixth round was canceled due to heavily bleeding wounds in the mouth of Corrales favor Casa Mayors. Due to the controversial circumstances occurred on 6 March 2004 for direct rematch. This time, Corrales won just on points and was awarded the vacant WBO title, which had been previously laid down by Freitas due to a weight class ascension.
Then moved Casamayor in the lightweight and defeated the undefeated Puerto Rican Daniel Seda ( 20-0-1 ) on points. In the fifth round, both boxers were on the ground. Within a short time Casamayor was given the opportunity to fight for a world title; on December 4, 2004, he ran against WBC titlist Jose Luis Castillo, which he, however, scarce and controversial subject on points. In his next bout, he boxed in June 2005 against the undefeated Olympian Kyrgyzstan Almazbek Raimkulow undecided.
On October 7, 2006, there was then the third clash with Diego Corrales, who held the WBC belt was now at lightweight. Corrales, whose planned third fight against Castillo had previously been canceled due to its weight problems, but now, ironically, could not even keep the weight limit for this fight, so that he was stripped of the title after weighing. Casamyor remained within the weight limit and could therefore still win the title now vacant explained what he finally succeeded with a narrow points victory. In February 2007, awarded the title but has been revoked because he did not fulfill his mandatory defense against David Díaz and instead a unification fight against Freitas planning, which ultimately did not materialize.
Casamayor defeated in November 2007 Jose Armando Santa Cruz narrowly and disputed on points, convinced, however, in March 2008 with a victory by technical knockout in the tenth round against the undefeated Australian Michael Katsidis. On 13 September 2008 he was defeated, however, against Juan Manuel Marquez for the first time in his pro career prematurely after two knockdowns in the eleventh round of the fight was stopped by the referee.