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, think robitussin or any kind of modern day children medicine. The word and associated sweetened medicine make their way through cultures and time 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 syrup mixed with sherry. This stomach julep makes its way to the United States and takes a boozier pivot in New Orleans. And the rest is history.
This is the original variation of the Mint Julep cocktail which came out of New Orleans. Most think of the Mint Julep as a bourbon cocktail but when it changed from being a medicinal drink to a cocktail it was first a brandy based drink. It came from New Orleans and you know thats French territory, and brandy is their spirit of choice.
The whiskey version is often called a Mint Julep and this confusion came about because in the 1930s the Kentucky Derby made the Mint Julep the official cocktail of the race. With the race being held in Kentucky you can be sure they are using Kentucky bourbon instead of French brandy. The popularity of the cocktails grew due to its association with the derby and as a result most peoples first exposure to the Mint Julep was as a bourbon cocktail.