TV Guide Magazine: Hell, yeah!
Baker: All those times we heard the voice of Red John? That was me.

TV Guide Magazine: Seriously?
Baker:
Yes! In the Season 2 finale, when Jane was captured and Red John did
that "Tyger, Tyger" thing with the mask on? That was me. I played both
characters in that scene.

http://www.tvguide.com/News/Mentalist-Simon-Baker-Red-John-1073837.aspx