William J. Fallon (January 23, 1886 – April 29, 1927) christened The Great Mouthpiece by the press was a prominent defense attorney during the 1920s who defended the gangster Arnold Rothstein and his accomplice Nicky Arnstein during the trial for the fixing of the 1919 World Series.

In 1918 he set up a law firm with his friend, Eugene F. McGee; it was around this time that he met Arnold Rothstein who was taken by the polished charm of the lawyer who was known for his oratory skills taught to him during his Jesuit education and his reputed photographic memory that allowed him to memorize whole books in hours.

[2] During the Black Sox Trial Fallon had a falling out with his client Nicky Arnstein because of his noticeable heavy drinking.

[4] Four years after his death a popular biography was written by Gene Fowler was published, which inspired the Warner Brothers films The Mouthpiece (1932), The Man Who Talked Too Much (1940) and Illegal (1955).

He is also portrayed for six episodes by David Aaron Baker in the HBO television series Boardwalk Empire.