From a Grub Street interview with Imbibe! author David Wondrich:

There’s new material on drinks in the new book, particularly the history of the Julep, which you say is a much older drink than previously thought.
It’s a much earlier drink. In 1770, in Virginia, there are two solid references to the julep being a recreational drink. That’s a big deal, I think. I had looked at the part on the julep in the original edition and I was shocked and disappointed. I wrote almost nothing about it. I wanted to kick myself, because that’s the most important drink.

You call it the “first true American drink.”
It’s a foundational drink. It’s how we started to be different. The mint julep is also the only drink that I’ve championed that hasn’t been revived yet.

Really? People make nice mint juleps at many places.
Some. Not so much. Nobody really specializes in them. People will make them if you ask.

And it was a brandy drink originally?
In the 1700s, it was a rum drink. The Revolutionary War years and a little after, a whiskey drink. Once the country got rich again and started making money again, it was a brandy drink, up until the Civil War.