The history of this mighty cocktail, and Juleps in general, goes way way WAY back to ancient Persia (modern day Iran). Rosewater was thought to have health benefits and the word for rosewater is gulab (gul=rose, ab=water). The word makes it way to the surrounding Arab culture and the word Julāb was used for any kind of sweetened medicinal syrup, kinda like robitussin. The word and associated sweetened medicine make their way through cultures and eventually make it to England as Julepum or Julep. In the 1750s a Julepum Stomachicum (a stomach julep to settle an upset tummy) is published in medical journals and calls for mint infused sweetener mixed with sherry. The stomach julep makes its way to the United States and takes a boozier pivot in New Orleans. And the rest is history.
While the Juleps are heavily associated with the Kentucky Derby, this drink actually came out of New Orleans. Keep in mind that there is a Julep for every kind of spirit out there, whiskey, gin, brandy, rum, Champagne, etc. if you look the juleps up in old books they would specify the gin julep as being made the exact same as the mint julep but with gin instead of brandy