Hello.

I'm afraid that your question isn't a very good one, because 'millionaire' isn't really a remarkable distinction, much though you and I might aspire to be one.

In American terms, many of the Founding Fathers had a net worth valued well into the millions - George Washington alone had a fortune that is estimated to have exceeded $500 million. Thomas Jefferson owned 'Monticello', a sprawling, 3,000 acre plantation that was surely valued. So in this sense, there were already quite a few millionaires in the Americas before 'USD' was even a thing.

Likewise throughout history, any ruler of note could've called upon extensive resources. Asking who the first millionaire in history was is a little like asking whose the first person to tell a joke.

Now if you want to refine your question to the first USD billionaire, then I believe the answer to your question is John D. Rockefeller.