Ali, who started his stellar boxing career under his given name of Cassius Clay, morphed from a promising young fighter to the face of a sport and one of the leading political voices of his generation in front of millions of Americans, who watched his classic fights against top fighters including Sonny Liston, Joe Frazier, George Foreman and many others.
At this point, decades after his retirement from the ring, Ali is remembered as one of the most important, divisive and dominating athletes in American, even world, history, so it is only fitting that a whole new generation of admirers are taking to the Web to pay their respects to him.
Many of the greetings came via Twitter, and Ali's name trended on the social media site for much of Thursday morning as users tweeted and retweeted thousands of messages about the inimitable boxer.
Mike Tyson, another of the best, most controversial boxers in the history of the sport, tweeted out his own special birthday greeting for Ali, which was retweeted nearly 2,000 times by noon Thursday:
"Happy Birthday to Muhammad Ali who shares this special birthday with my mentor, Cus D'Amato," Tyson wrote on his official Twitter feed.
"Happy Birthday to The Greatest, my father Muhammad Ali!"
Some other Twitter users have just expressed their gratitude and admiration for an athlete who inspired so many people both inside and outside of the ring.
"Happy 71st birthday to the greatest sports person of all time -Muhammad Ali."
And the birthday wishes, tributes and superlatives will continue to pour forth online throughout the day on Thursday, as folks take the time to honor "The Greatest."