This served for Hagler to build upon a character that was as tough as steel, but with a soft spot in his heart toward his fans. Hagler avenged his 2 defeats by knocking out Monroe and Watts in rematches, and won a 10 round decision over Bennie Briscoe in a classic Philadelphia confrontation, and by this time he had made fans in both Massachusetts and Philadelphia. By then, promoter Bob Arum, a lawyer who had helped in the White House during President John F. Kennedy's tenure, took notice and signed him. Hagler needed some gubernatorial help, but Arum pulled the strings, and finally, in November 1979, Hagler was in the ring with a world Middleweight champion. Vito Antuofermo gave Hagler a shot, and the fight turned into a Middleweight classic. One of the greatest championship bouts in Middleweight history was unfortunately marred by controversy, and Hagler had to settle for a draw or tie. This only added to Hagler's frustrations.
Antuofermo lost his title later to Alan Minter, who became the 2nd champion to be brave enough to defend his title against the Brockton menace. Hagler went to London and beat Minter in 3 rounds at the Wembley Arena. At the conclusion of this bout a riot broke out, and Hagler and his trainers had to be carried away to their locker rooms by the police, in the middle of a rain of beer bottles and glasses.
Hagler proved a busy world champion and he defeated future world champion Fulgencio Obelmejias by a knockout in 8, and then former world champ Antuofermo by knockout in 5, both at the Boston Garden. Mustafa Hamsho, who would later defeat future world champ Bobby Czyz followed, beaten in 11. Caveman Lee lasted only 1 round, and in a rematch in Italy, Obelmejias lasted 5. British champion Tony Sibson followed Hagler challengers' trips to the canvas, falling in 6, and Wilford Scypion went in 4. By then Hagler was a staple on HBO which was the Pay Per View of it's time. A super-fight vs Roberto Duran followed and Duran was the first challenger to last the distance with Hagler in a world championship bout, but he lost a unanimous 15 round decision. Then came Juan Roldan, who became the only man to drop Hagler, scoring a knockdown seconds into the fight, but Hagler got angrier and proceeded to beat Roldan, stopping him in 10. Hamsho was given a rematch and lost in 3, and then in April 15th, Hagler and Thomas Hearns met in what was billed as The War. In a sensational slugfest, Hagler survived one of the best first rounds in history and won by an electryfing 3 round knockout. Next was John "The Beast" Mugabi,a thunderous puncher who was 26-0 with 26 knockouts. Hagler took Mugabi's best shots and came back handily, stopping Mugabi in the 11th in what would turn out to be his last successful defense. Hagler's next challenger was Sugar Ray Leonard, who won a close split decision in Las Vegas April of 1987.
After his defense vs Hearns and before his fight with Mugabi, Hagler changed his name legally to Marvelous Marvin Hagler and made a few commercials, most notably a commercial for Pizza Hut. He enjoyed his fame and relished the fact that after so many years in the background, he was finally a household name.
He thought his decision loss to Leonard was undeserved, and quit boxing tired of the backroom politics of the sport. He moved to Italy, where he made movies, including playing a US Marine in the movie Indio.
He is now a member of the International Boxing Hall Of Fame.