Al Schacht, who entertained millions of fans as the Clown Prince of Baseball, died Saturday in Waterbury (Conn.) Hospital after a brief illness.

He was 91 years old.

Born in New York City on Nov. 12, 1892, Alexander Schacht did not start out to become a baseball zany. A sore arm cut short his major league career as a pitcher with the Washington Senators, which lasted from 1919 to 1921. He won 14 games and lost 10.

After a stint in the minors, during which he failed to rehabilitate his arm, Mr. Schacht turned to clowning in 1924. He teamed with Nick Altrock, and they made nearly everybody happy with their capers except each other.

After causing fans to break up with laughter for 10 years, the two broke up their act. Mr. Schacht went on to gain a world reputation as a pantomimist.

The first time he drew a roar was in 1914, as he was pitching for the Newark Bears of the International League. He made his way to the mound astride an old nag. The horse barely made it, and Mr. Schacht had trouble, too. By the second inning, he was on his way to the showers, walking, not riding.

He entertained in many World Series and All-Star Games, and in nearly every city in both the majors and minors. He brought happy moments to troops in Europe, Africa and Asia during World War II. Mr. Schacht later entered the restaurant business and wrote his autobiography, ''I Always Had More Fun Than Anybody.''

A service will be held at 2 P.M. today at the Munson-Lovetere Funeral Home, Main Street, Southbury, Conn. Mr. Schacht, who had no children, is survived by his wife, Maybelle; his brother, Lawrence, and his sister, Esther.