A national holiday honoring King Jr. is also being observed Monday.
Nicknamed, "The Greatest," Ali was born Cassius Marcellus Clay in 1942, in Louisville, Ky., and dominated the sport throughout the 1960s and 1970s with the speed and athleticism of a middleweight.
Ali's self-described style was to "float like a butterfly and sting like a bee," as well as the "Rope-a-Dope" strategy that led to a stoppage victory over George Foreman. Ali was always a vocal opponent who often goaded his rivals with rhymes and trash talk.
A three-time titlist over a 15-year-span, Ali, as Clay, won an Olympic gold medal in Rome in 1960 and earned his first belt by defeating Sonny Liston by seventh-round knockout in 1964.
Clay changed his name later that year after joining the Muslim Nation of Islam, and subsequently had his title revoked after being sentenced to five years in prison for draft evasion -- even as Ali cited his newly discovered faith as cause for his refusal to serve in the Vietnam War.
Ali resumed his career following the Supreme Court's reversal of his conviction in '71 and soon commenced a three-fight series with then-champ Joe Frazier.
Ali was floored once in their first bout in March '71, but won the next two in January '74, and, October '75, respectively, the latter by 14th-round knockout in "The Thrilla In Manilla" when the late trainer, Eddie Futch, refused to allow Frazier to leave his stool for the final round.
"Muhammad Ali was the premiere attraction in boxing in the '70s after his exile, where everybody in the world felt that he was not only a great athlete and a great fighter, but almost like a prophet because he was the first public figure to openly espouse his distaste for the Vietnam war," said Top Rank Promotions CEO Bob Arum, who handled Ali in the third bout against Frazier.
"And, so, he was like larger than life. And the Philippines wanted to do this fight because they were having a Muslim insurrection, and so they arranged for this fight to happen," said Arum. "The country went nuts about Muhammad Ali and were glorified with this fight. It was a major event for Philippines recognition."
Ali regained the heavyweight title -- this time, the WBA and WBC belts -- for the second time by an eighth-round knockout of Foreman in Oct. 1974's "Rumble In The Jungle," in Kinshasa, Zaire.
Ali earned the WBA version of the crown for the third time by dethroning former Olympic gold medalist Leon Spinks in their return bout in Sept. '78 to avenge a loss of the WBA and WBC belts seven months earlier.
Ali retired after losing to Trevor Berbick in Dec. 1981 with a mark of 56-5, including 37 knockouts.
"Muhammad Ali is a towering social and political figure. He belongs to the world," said noted boxing historian, Thomas Hauser, author of Muhammad Ali: His Life And Times. "But he also belongs to boxing, and we should be very proud of that."