He began as an Irish immigrant's son studying for the Catholic priesthood at Boston College, then became the highest-paid athlete of his era and arguably the world's first true sports superstar. John L. Sullivan, the Boston Strong Boy, was the bridge from secret illegal bare-knuckle brawls to the bright lights of modern professional sports.

Sullivan weaponized the transcontinental railroad and telegraph to build a national brand, offering cash to any man who could last four rounds. His 75-round bare-knuckle war with Jake Kilrain marked the end of one era, while his gloved loss to Gentleman Jim Corbett ushered in the modern one. He later became a sober prohibition lecturer.

• The pivot from seminary to professional baseball to illegal prizefighting
• The grueling 1883 to 1884 cross-country tour that built modern sports marketi ...