World War II soon broke out all over Europe, however, and D' Agata had to wait until he was 20 years old to fight as an amateur. In 1946, he began an amateur career that saw him win 90 out of 110 bouts. On October 14 of 1950, he turned professional, defeating Giuseppe Saladari by decision in six rounds.
D' Agata built a record of 10-0 with 1 knockout before suffering his first loss, at the hands of Romolo Re, August 2 of 1951, by a decision in ten. Another loss to Re would follow, but D' Agata was able to build a 19-3-2 record with 4 knockouts before challenging for the Italian Bantamweight title. On September 27, 1953, he beat Gianni Zuddas by a disqualification in round nine to earn that belt. After five more wins, he travelled to Tunisia, where he met the future world champion Robert Cohen, losing a ten round decision. After two more wins, he embarked on what would have been a long tour of Australia—a trip cut short after three wins when he was shot by his associate.
D' Agata then returned to Italy, where he was no longer recognized as champion. On May 25, 1955, he defeated Arthur Emboule by decision in eight rounds, in his first bout after the shooting. After eight more wins in a row (which raised his winning streak to a total of thirteen wins in a row), he was given a shot at the European Bantamweight title by Andre Valignat on October 29 of the same year, and he defeated Valignat by a fifth round disqualification.
On June 29, 1956, D' Agata finally received his world title opportunity, when former conqueror Cohen gave him a chance to win the world Bantamweight title in Rome. D' Agata made his dream come true by knocking Cohen out in six rounds in front of 38,000 fans, many of whom rushed to the ring the moment the fight was over, carrying D' Agata out of the ring on their arms. With that win, D' Agata made history as the first deaf world champion of boxing.
D' Agata went to Paris to defend his title for the first time, on April 1, 1957, against local challenger Alphonse Halimi. Special lights had been set specially for this fight, so that D' Agata, who could not hear the bell after each round, would be able to tell when each round was finished. These lights would flash the moment the bell rang. There was a storm on the day of the fight, which was held in an open air area. Lightning struck one of the special lights in round three, and D' Agata was struck by sparkles, suffering a burned neck and back. It was decided the fight would go on, and D' Agata tried to defy the odds for the remaining of the fight, but he lost the title by a fifteen round decision.
D' Agata never received a rematch from Halimi. He then fought on with mixed success, until August 1, 1962, when he announced his retirement. He was able to stay away from boxing for the rest of his life.
D' Agata had a record of 54 wins, 10 losses and 3 draws, with 22 wins by knockout.