How many times in his career did Ali lose a professional boxing match?

Answer 1

5 times.

As a professional, Muhammad Ali officially lost his first fight by unanimous decision to Joe Frazier in 1971. His second defeat was via split decision to Ken Norton in 1973. He would defeat each of those men twice in subsequent bouts (although with notable controversy in his final bout with Norton in 1976)

In 1978, he lost by split decision in a monumental upset against Leon Spinks, but managed to avenge the loss 7 months later before retiring.

This would have left him with 3 career defeats. However, Ali chose to come out of retirement and, as barely a shell of his youth, he was soundly defeated by Larry Holmes and later by Trevor Berbick, which brought the total to 5 defeats overall.