Michael Pataki - TOS's Karnas & TNG's Korax
You got it reversed. Pataki was Korax on TOS and Karnas on TNG.
I like finding actors in performances before
they did Star Trek. The most well-known occurance is probably Shatner and Nimoy appearing in the same episode of The Man from U.N.C.L.E., a full year before Kirk and Spock. Shatner, Nimoy, and James Doohan all appear in episodes of the original Outer Limits, as well as several future guest stars. Nimoy shows up in an early episode of Bonanza ("The Ape") as a slimey cardshark, available for free download at the Internet Archive.