Since July 18, 2005, he has been rated by The Ring magazine as the number-one pound-for-pound boxer in the world. Floyd Mayweather Jr. has won four world boxing championships in four different weight classes, and he is the current IBF welterweight champion.

Mayweather Jr. is a member of the Mayweather boxing family. His father, Floyd Mayweather Sr., is a former welterweight contender. One of his uncles, Jeff Mayweather, is a former IBO super featherweight champion. Another uncle, former two-division world champion Roger Mayweather, is Mayweather Jr.'s current trainer.

Mayweather had a successful amateur career, with a record of 84-6. He won national Golden Gloves championships in 1993 (at 106 lb), 1994 (at 112 lb), and 1996 (at 125 lb). He was given the nickname "Pretty Boy" by his amateur teammates because his face never had cuts or bruises after fights a result of the defensive techniques that his father (Floyd Mayweather Sr.) and uncle (Roger Mayweather) had taught him. In his orthodox defensive stance, Mayweather holds his right hand high (near his right temple), cradles his stomach with his left hand, and guards his chin with his left shoulder. From this stance, he blocks, slips, and deflects most of his opponents' punches, even when cornered, by twisting left and right to the rhythm of their punches.

At the 1996 Olympics in Atlanta, Mayweather won a bronze medal by reaching the semifinals of the featherweight (57 kg) division's 31-boxer tournament. In the opening round, Mayweather led 10-1 on points over Bakhtiyar Tileganov of Kazakhstan before he won by round 2 referee stoppage. In the second round, Mayweather outpointed Artur Gevorgyan of Armenia 16-3. In the quarterfinals, Mayweather survived a late rally by Lorenzo Aragon of Cuba to win 12-11. In his semifinal bout against the eventual silver medalist, Serafim Todorov of Bulgaria, Mayweather lost by a controversial decision that the U.S. team officially protested. Nearly everyone who saw the bout, including the referee, thought that Mayweather had won. However, the international judges, whom U.S. boxing coach Al Mitchell called "a bunch of mobsters," ruled the bout 10-9 for Todorov.

Mayweather fought his first professional bout on October 11, 1996 against fellow newcomer Roberto Apodaca. Mayweather won by knockout in round 2. Mayweather's trainer at the time was his uncle, Roger Mayweather, because Floyd Mayweather Sr. was still imprisoned after having been convicted of illegal drug trafficking in 1993. Mayweather Sr. took over as Mayweather Jr.'s trainer when he was released from prison (after Mayweather Jr.'s fourteenth fight a second round knockout of Sam Girard).[8] From 1996 to early 1998, Mayweather fought against relatively easy opponents and won most of the fights by knockout or TKO. The most notable of these fights was a unanimous decision victory over former IBO lightweight champion Tony Pep on June 14, 1998.

In 1998, Mayweather won his first world title, the WBC super featherweight (130 lb) championship, when the corner of Genaro Hernandez stopped the fight after round 8. From there, Mayweather defended his title with dominating performances against contenders such as Angel Manfredy and Carlos Gerena. Mayweather was named as The Ring magazine's fighter of the year for 1998.

Before he fought against former WBC featherweight champion Gregorio Vargas in early 2000, Mayweather fired his father as his manager and replaced him with James Prince. A few months after the fight, the rift between the father and son became wide enough that Mayweather Jr. fired Mayweather Sr. as his trainer as well. Roger Mayweather returned to his role as Mayweather Jr.'s trainer in his next bout a non-title fight against Emanuel Burton. In an interview in 2004, Mayweather Jr. said that he loves Mayweather Sr. as his father but feels that he has better chemistry with Roger, and his father had put too much pressure on him to be perfect.

Mayweather's biggest fight as a super featherweight was on January 20, 2001, against Diego Corrales. At the time, neither fighter had been defeated or knocked down. In the bout, Mayweather won every round and knocked down Corrales five times (three times in round 7 and twice in round 10). After the fifth knockdown, Corrales' cornermen climbed onto the apron and stopped the fight, thereby establishing Mayweather as one of the claimants to boxing's mythical pound-for-pound title.

In Mayweather's next bout, on May 26, 2001, future IBF champion Carlos 'Famoso' Hernandez "knocked down" Mayweather for the first time. Mayweather entered the bout with injured hands. When Mayweather hit Hernandez with a left hook in round 6, the pain caused Mayweather to drop his left hand to the canvas, and the referee called it a knockdown. Nonetheless, Mayweather won the fight by unanimous decision. In the award-winning documentary film More Than Famous, Hernandez's bout against Mayweather was prominently featured.

Mayweather's last fight in the super featherweight division was against future super featherweight and lightweight champion Jesus Chavez. It was Mayweather's eighth defense of the WBC super featherweight title, which he had held for more than three years. He won when Chavez's corner stopped the fight after  round 9. Mayweather had such difficulty making weight for this fight that he did not eat for four days before the weigh-in.

In 2002, Mayweather moved up to the lightweight (135 lb) division. Mayweather fought only four bouts at this weight, but they were all world championship fights.

Mayweather won two close bouts for the WBC and The Ring lightweight belts against Jose Luis Castillo. In their first bout, Castillo had success when he cut off the ring and used his strength to wear down Mayweather. Some analysts feel that Mayweather should have lost the fight, but he won by unanimous decision. In the less controversial rematch, Mayweather used his quick footwork and combinations to coast to another unanimous decision victory.

On April 19, 2003, Mayweather dominated the Dominican Victoriano Sosa and won by unanimous decision. Mayweather's next fight (on November 1, 2003) was in his hometown of Grand Rapids, Michigan. He fought against the promising South African knockout specialist Phillip Ndou, whose record was 31-1 with 30 KOs. Uncharacteristically, Mayweather was offensively oriented from the beginning of the fight. Round 5 was one of 2003's most action-packed. In the middle of the round, Mayweather landed a barrage of powerful punches. Ndou endured and threw wild punches that forced Mayweather into the ropes, but Mayweather demonstrated his rhythmic defensive technique and let Ndou wear himself out further. In round 6, Ndou wobbled and was pushed down. In round 7, a combination of three straight right hands knocked down Ndou and caused a TKO.

Mayweather then moved up to the super lightweight (140 lb) division. His first fight in this division was against southpaw DeMarcus Corley. Mayweather used his speed to win the early rounds. In the first minute of round 4, Corley landed a solid right hand and drove Mayweather into the ropes, but Mayweather recovered and fought back ferociously. After that round, Mayweather mostly controlled Corley. Mayweather knocked down Corley in rounds 8 and 10, but Corley was able to continue until the end. Mayweather won by unanimous decision. The fight was Mayweather's only one in 2004.

On January 22, 2005, Mayweather fought against Henry Bruseles of Puerto Rico in a WBC super lightweight title eliminator bout. Mayweather easily outclassed Bruseles throughout the first seven rounds. In round 8, Mayweather knocked down Bruseles twice, and the fight was stopped.