Muhammad Ali was a boxer.
Muhammad Ali ( [ ɑːˈliː ] born Cassius Marcellus Clay Jr. ; January 17 , 1942 -- June 3 , 2016 ) was an American professional boxer and activist . Between February 25 and September 19 , 1964 , Ali reigned as the undisputed heavyweight champion . In professional boxing , the undisputed champion of a weight class is a boxer who is recognized as the world champion at that class by each boxing organization .