Marvelous Marvin was the undisputed World Middleweight Champion from 1980 to 1987. He retired with a career record of 62 wins, 3 losses, and 2 draws, with 52 wins by knockout. He was inducted into the International Boxing Hall of Fame in 1993. He became a Laureus World Sports Academy Member in 2007.

He spent his early years in Newark, but during the 1967 race riots, in which 26 people died, the family's tenement was destroyed and the Haglers moved to Brockton in Massachusetts. In 1969, he walked into a gym owned by brothers Pat and Goody Petronelli and fell in love with boxing. They became his trainers and managers, and he embarked on his remarkable career.

Marvelous Marvin earned his first shot at the undisputed middleweight crown in November 1979, but he could only draw with champion Vito Antuofermo. He could not be resisted long, however, and in September 1980, he beat the new champion, Britain’s Alan Minter, in three rounds to win the title which he held for seven years.

Hagler made 12 successful title defences.  Among his victims were Vito Antuofermo, Mustafa Hamsho, Roberto Duran, Juan Roldan, John Mugabi, and Thomas Hearns.  His three-round shootout with Hearns is regarded as one of the best fights of all-time. 

As soon as the opening bell sounded, Hagler rushed forward and attacked, abandoning boxing for a wide-open melee.  Hearns, who had planned to use his height and reach, was caught off-guard and forced to trade punches at close range with the stronger Hagler.  After some first round success by Hearns, who had opened a nasty cut on Hagler's forehead, the champion took control. 

Midway through the third round, knowing that the referee was close to stopping the fight and awarding Hearns victory, Hagler sent Hearns reeling across the ring with a right.  Hearns had not even regained his balance before Hagler hit him again. Although Hearns unsteadily climbed to his feet at the count of nine, the fight was stopped to give Hagler the defining moment of his career.

Hagler's last fight was in 1987 when Sugar Ray Leonard came out of retirement and won an exciting, but controversial 12 round split decision for the WBC middleweight title. Hagler called for a rematch, but Leonard refused.

Following his retirement, he then moved to Italy and began a career in Italian action films. His roles include a US marine in the films Indio and Indio 2. In 1996, he starred in Virtual Weapon, alongside Giselle Blondet. He still lives in Italy and has made visits to several Laureus-supported programmes there.