On 20 January 1961, Benvenuti made his professional boxing debut, beating Ben Ali Allala by decision in six rounds. He then won 29 fights in a row before challenging for the Italian Middleweight title, on 1 March 1963, in Rome against Tommaso 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 Junior Middleweight champion Denny Moyer on 18 September 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 18 June 1965. This was a fight the Italian public clamored 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 junior 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 15 October 1965, he added the European belt at the Middleweight division, with a sixth-round knockout of Luis Folledo.
A rematch with Mazzinghi took place on 17 December 1965, and Benvenuti retained the world junior middleweight crown with a fifteen-round decision. 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 junior middleweight title against Ki-Soo Kim, who won by decision in fifteen rounds on 25 June 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 17 April 1967, Benvenuti beat Emile Griffith by decision in fifteen rounds at New York City's Madison Square Garden, in what was the beginning of their trilogy of fights, to win the world middleweight title. On a rematch at Shea Stadium on 29 September 1967, he lost by a decision in fifteen rounds.
On 4 March 1968, Benvenuti and Griffith completed their trilogy, once again at Madison Square Garden, with Benvenuti knocking Griffith down in round nine and winning a fifteen-round decision to regain the world Middleweight title. On 14 December 1968, in San Remo, he and Fullmer met once again, and Benvenuti retained the world middleweight title with a fifteen-round decision. On 26 May 1969, Benvenuti was outpointed over ten rounds by world Light Heavyweight champion Dick Tiger in an over-the-weight, non-title match.
The most curious defense of Benvenuti's active reign, took place on 4 October 1969, when he retained the world Middleweight title with a seven-round disqualification win over American Fraser Scott at the Stadio S. Paolo in Naples. From the first round, Scott was warned repeatedly, and with increasing intensity from the referee, about attempted butting. Scott, a young fighter unschooled in the European insistence on what his trainer referred to as "that...Olympic stand-up style", knowing only the battle plan he went in with and speaking no Italian, did not understand the warnings at first, then was unable to alter his approach; to the American, he was merely "ducking" Benvenuti's shots. The bout was foul-filled even without this added controversy; Scott would later accuse Benvenuti of having tried to thumb him, and during the sixth round, the fighters' legs became entangled as they wrestled, causing both to crash to the canvas. Round Seven saw the stoppage, the referee asserting "attempted butting", Fraser Scott and corner forever insisting he had "ducked".
On 22 November 1969, he beat former world Welterweight champion Luis Rodriguez by knockout in 11 rounds to, once again, retain his world Middleweight title.
This marked the beginning of a downfall period for Benvenuti: In his next fight, on 13 March 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 Monzón by knockout in round twelve in Rome on 7 November 1970.
In 1971, 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 8 May 1971, in Monte Carlo. He announced his retirement there, and never returned to boxing. Benvenuti had a record of 82 wins, 7 losses and 1 draw (tie) in 90 professional boxing bouts, with 35 wins by knockout. In 1992 he was inducted into the International Boxing Hall of Fame.
Nino Benvenuti came to life in the picturesque town on Isola d'Istria (once Italian, now Slovenia). After the war his family fled to Italy due to the consequences of the war treaty and the hostilities created by the Yugoslav government.
In 1961 Benvenuti married Giuliana Fonzari; they had four sons (Stefano, Macri, Giuliano and Francesco) and adopted a Tunisian girl (Soraya). They later divorced, and Benvenuti married Nadia Bertorello, with whom he had one daughter (Nathalie).
After retiring from boxing Benvenuti became a successful businessman, show host and city counselor in Trieste. He opened a high-class restaurant and forged friendships with Monzon and Griffith. In 1980 he asked Griffith to become the godfather of one of his sons, and later helped him financially. Monzon was a guest of honor at Benvenuti's television show several times, and, when he was accused 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.I, Benvenuti. Carroccio. 1967.
Nino Benvenuti (2001). Il mondo in pugno. Sperling & Kupfer. ISBN 978-88-200-3074-2.