It most certainly was already a thing in the Roman Empire. Charioteers were very well paid athletes. Very much on par (in equivalent wealth) with the wealthiest of modern athletes.

There have been some (dubious because difficult) comparisons with modern athletes that essentially came down as 'roman charioteer had a higher career income than any modern athlete'. Take it with a few barrels full of salt, though. That said, some of them were filthy rich.

E.g.: The Careeor of Diocles, Roman Charioteer by Robert B. Kebric (Univ. of Louisville)

In Greece, there were professional trainers (for those who wanted to compete in athletic competitions, so that was a career option for succesful athletes). But they didn't seem to see athletics itself as a thing you were supposed to earn money with; Athletics in Ancient Greece and Greek Athletics. They did have prizes though, and favorites of a particular city state might be subsidized by other locals; there were certainly people who dedicated their full time to athletics.

I won't comment on older civilizations than that; maybe somebody else can shed some information on those.

Hello, after reading the thread, I notice a pattern where all answers mention Europe. Does that mean that professional athleticism was a phenomenon only in Europe or did it develop outside of Europe too? (pre-European contract, of course)