Paul Mercier provided the voice of Leon S. Kennedy in the original Resident Evil 4.
Mercier voiced Leon in the 2005 game, giving the character a more experienced and sarcastic personality than in the earlier Resident Evil 2 appearance. Leon’s mission in Resident Evil 4 takes place six years after the Raccoon City incident.
The voice cast changed in later versions and adaptations. Matthew Mercer voiced Leon in the 2012 animated film Resident Evil: Damnation and in the 2019 remake, while Nick Apostolides voiced Leon in the 2019 remake. Those later performances are why the names are often confused.
Paul Haddad, who voiced Leon in Resident Evil 2, is another distinct performer. The question refers specifically to the original 2005 Resident Evil 4, not its remake or animated films.