In the early 1900s, the world's most popular sport was, by far, boxing. And in the pecking order of boxing, the heavyweight champ was the most popular of all boxers. In that period, the dominant heavyweight was Jack Dempsey. For a decade, he was easily the most famous man on earth.
He had the first five bouts with $1M+ gate receipts in boxing history, including his 1927 rematch w/ Gene Tunney that drew 105,000 to Soldier Field with a gate receipt of $2.7M (approx $40M today), a record that stood for over 25 years.
Edit: typo
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