He won 29 fights in a row before challenging for the Italian Middleweight title, on March 1, 1963, in Rome against Tomasso Truppi. His winning streak extended to 30 when he knocked out Truppi in round eleven. His winning streak reached 46 wins in a row when he met former world Jr. Middleweight champion Denny Moyer on September 18, 1964, beating Moyer on points in ten rounds.
After reaching 55 wins in a row, including a five round knockout of Truppi in a rematch, he met world Jr. Middleweight champion Sandro Mazzinghi in Milan, on June 18, 1965. This had been a fight the Italian public had been clamoring for: Both men were Italian, both men claimed to be the best in their division, and they had expressed desire to fight each other. Benvenuti became the world Jr. Middleweight champion with a sixth round knockout win. It was common, at that era, for world champions to fight for regional belts after winning the world title, so on October 15, he added the European belt at the Middleweight division, with a sixth round knockout of Luis Folledo.
A rematch with Mazzinghi took place on December 17, and Benvenuti retained the world Jr. Middleweight crown with a fifteen round decision. But after three non title wins, including a twelve round decision over Don Fullmer and a fourteen round knockout in Germany of Jupp Elze (Benvenuti's first professional fight abroad), he travelled to South Korea, where he lost his world Jr. Middleweight title against Soo Kim Ki, who beat him by decision in fifteen rounds on June 25, 1966. Benvenuti deemed his first loss as a professional as an unjustified decision for the local boxer, and, frustrated by it, he decided to concentrate on the Middleweight division instead.
On April 17, 1967, Benvenuti beat Emile Griffith by decision in fifteen rounds at New York's Madison Square Garden, in what was the beginning of their trilogy of fights, to conquer the world Middleweight title. But on a rematch at Yankee Stadium on September 29, he lost by a decision in fifteen, and the world Middleweight title belt. Also in 1967, he wrote his auto-biography book, I, Benvenuti.
On March 4, 1968, Benvenuti and Griffith completed their trilogy, once again at the Madison Square Garden, with Benvenuti dropping Griffith in round nine and winning a fifteen round decision to regain the world Middleweight title. On December 14, in San Remo, he and Fullmer met once again, this time with the world Middleweight title on the line. Benvenuti retained the belt with a fifteen round decision. On May 26, 1969, Benvenuti was outpointed over ten rounds by world Light Heavyweight champion Dick Tiger in an over-the-weight, non-title match. On October 4, he retained the world Middleweight title with a seven round disqualification win over American Fraser Scott in a foul-filled bout. On November 22, he beat former world Welterweight champion Luis Rodriguez by knockout in 11 to, once again, retain his world Middleweight title.
This marked the beginning of a downfall period for Benvenuti: In his next fight, on March 13 of 1970, he was knocked out in the eighth round of a non-title fight by unknown American Tom Bethea in Australia. While this upset defeat caused Bethea to get a world title shot at Benvenuti's title and Benvenuti avenged the defeat with an eighth round knockout of Bethea himself, Benvenuti soon lost his title for the last time, being beaten by young Carlos Monzon by knockout in round twelve in Rome on November 7.
In 1971, and after losing a ten round decision to Jose Chirino, Benvenuti got a rematch with Monzon for the world Middleweight title. Benvenuti was once again beaten by Monzon, this time by knockout in round three on May 8, 1971, in Monte Carlo. He announced his retirement there, and never returned to boxing.
Benvenuti became a successful business man, show host and city counselor in Trieste. He opened a high class restaurant and forged friendships with Monzon and Griffith, and he sent his son to take boxing classes with Griffith in New York. Monzon was a guest of honor at Benvenuti's television show several times, and, when he was acussed of murdering his wife in 1988, Benvenuti became one of his most loyal supporters, visiting him in jail in Argentina several times, and clamoring for Monzon's freedom.
Benvenuti had a record of 82 wins, 7 losses and 1 draw (tie) in 90 professional boxing bouts, with 35 wins by knockout.
He is a member of the International Boxing Hall of Fame.