Born Cassius Marcellus Clay Jr., he would later change his name to Muhammad Ali upon converting to Islam and affiliating with the Nation of Islam.

He was first directed toward boxing by Louisville police officer and boxing coach Joe E. Martin, who encountered the 12-year-old fuming after a thief stole his bicycle. When Cassius told Martin he was going to "whup" the thief, the officer suggested that he learn how to box first. Initially, Clay didn't take the idea seriously, but after seeing amateur boxers on a local television boxing program called Tomorrow's Champions, he decided to start training.

As an amateur, he won six Kentucky Golden Gloves titles, two national Golden Gloves titles, an Amateur Athletic Union national title, and the light heavyweight gold medal in the 1960 Summer Olympics in Rome.

In his 1975 autobiography, he wrote that shortly after his return from the Rome Olympics, he threw his gold medal into the Ohio River after he and a friend were refused service at a "whites-only" restaurant. Ali received a replacement medal at a basketball intermission during the 1996 Olympics in Atlanta, where he lit the torch to start the games.

He won the world heavyweight championship from Sonny Liston in a major upset on February 25, 1964, at age 22, becoming the youngest boxer to take the title from a reigning heavyweight champion. Soon after the fight, Clay changed his name to Cassius X, and then later to Muhammad Ali.

Ali's friendship with Malcolm X ended when Malcolm split with the Nation of Islam a couple of weeks after Ali joined. Ali later said that turning his back on Malcolm was one of the mistakes he regretted most in his life.

Ali was famous for his colorful quotations. He once said he should be a postage stamp because that was the only way he'd ever get licked.

Ali met "Gorgeous George" Wagner in Las Vegas in 1961. He later told the Associated Press, "I saw his aides spraying deodorant in the opponents' corner to contain the smell. I also saw 13,000 full seats. I talked with Gorgeous for five minutes after the match and started being a big-mouth and a bragger. He told me people would come to see me get beat. Others would come to see me win. I'd get 'em coming and going."

In 1966, Ali refused to be drafted into the military, citing his religious beliefs and ethical opposition to the Vietnam War. He was convicted of draft evasion, sentenced to five years in prison and a $10,000 fine, and stripped of his boxing titles. He managed to stay out of prison, appealing the decision to the Supreme Court (which overturned his conviction in 1971) but he wasn't allowed to fight for nearly four years--from ages 25 to almost 29.

In the late 60s, Ali and Rocky Marciano were the only undefeated heavyweight champions in history, and fans often debated who would win had they met in their primes. Ali and Marciano were filmed sparring for 75 one-minute rounds, simulating several possible outcomes. Punch-by-punch details of both boxers' records were entered into an NCR 315 computer, including their strengths, weaknesses, fighting styles, and other factors. Edited versions of the bout were shown in movie theaters in 1970, based on the computer's predictions. In the U.S. version, Ali lost in a simulated 13th-round knockout, but in the European version Marciano lost due to cuts, also simulated. Ali suggested that prejudice determined his defeat in the U.S. version, reportedly joking, "That computer was made in Alabama."