Cassius M. Clay changed his name in 1964 to Muhammad Ali when he became a member of the Muslim religion. He may be one of the most famous athletes, recognized worldwide. He earned several boxing championship titles including 1956 Novice Title, Louisville, Kentucky, 1959 & 1960 APR National Amateur Athletic Union Light-Heavyweight Champion, Golden Gloves, and 1960 Rome Olympics Light-Heavyweight Gold Medalist at the amateur level and 1964-67, 1974-78, and 1978-79 World Boxing Association heavyweight at the professional level. His first professional fight was at age 18.
His political and racial discrimination awareness led him to become more conscientious about world affairs, one of the reasons for his religious conversion. On religious grounds, in 1967, he refused to be inducted into the armed forces. Threatened with the penalties of incarceration and fines, he chose to stand by his personal values. His refusal caused him to be stripped of his world title in 1967 and he was barred from boxing while the case was in litigation. He continued to recognized as the world heavyweight titleholder in other parts of the world. In 1971, the U.S. Supreme Court ruled in Ali's favor.
With hard work since age 12, Ali was able to essentially resurrect the popularity of boxing through his confidence, talent, skill, boxing intelligence, and showmanship. By the end of his professional career, after twenty-one years in the ring, his record stands at 61 bouts with 56 wins—37 knock outs (usually in the round of his prediction), and 5 losses. Among other reasons, his Parkinson's disease influenced his decision to retire.
Though there were times controversy surrounded Ali, he continues to be a respected worldwide figure. In 1998, he was chosen to be a United Nations Messenger of Peace because of his work in developing countries. In 2005, Ali received the Presidential Medal of Freedom.
Whether you find Ali controversial or not, one thing that shouldn't be controversial is his courage to stand on his moral principles.
Compiled from various resources including Wikipedia.