In "Red Dawn," the 100th episode of the Mentalist, a mysterious man--later identified as Robert Kirkland of Homeland Security--sitting in a limo with FBI Regional Director, Alexa Schultz, appeared to have been following Patrick Jane
since Red John murdered his family. In "Red Sails in the Sunset," Bob Kirkland pursued Lorelei Martins, a Red John assistant, who revealed to Patrick Jane that he and Red John are much alike and she wondered how both didn't become best friends since the moment they shook hand. Evidence surfaced suggesting Bob Kirkland--who shook hands with Jane in a previous episode--is Red John. However, Jimmy Gadd, an editor on the Mentalist, confirmed that Bob Kirkland is NOT Red John and that Bob K. is a good guy with a story to be revealed in a future episode.

Ive Heard alot of interesting theories but this is what I believe

It is not
Patrick Jane
Van Pelt
Rigsby
Ray Haffner (too obvious after red barn)

However I also believe that season five wont get us anywhere closer to finding red john, The name isnt on the list the options are too open and he has confessed to missing some people and it has to be after his wife died.

All I hope is that this doesnt end like lost and Patrick Jane is actually delusional and this never happened. So Heller if you read this do not end it like that, I will be extremely pissed off. 

Hi, I'm an italian fan of The Mentalist and I want to show my theory about Red John. I think he is CBI Ron: is not my only theory but it is the one in which I believe more.

Ashley Gable (and not B. Heller, as many people said) said: "Yes, Red John has been visible in prior Episodes. You've got to look for him like Waldo". Well, you know, Ron is like Waldo: he is always there and sometimes he interacts kindly. But this is the trick: you think he is too insignificant and kind to be Red John, but this is the bright side of him. His "red side" is shown in the background. I can prove it!
But first, one thing: the first time we can clearly see Ron's face is the 1x07 episode; and the title of the episode is "seeing red". Bruno, you damned genius!

Now, the background: I bring you to the 2x07 episode, min. 04:30:
Rebecca gives to Bosco a file, than she goes away and we look at the team. But Rebecca is one of Red John's disciples, so we should follow her. She go away, but as soon as she get close to Ron, she immediately changes direction and is waiting for him outside the room (min. 05:27).
In the next episode Bosco's team will be murdered.

It's not enough: beginning of episode 3x08: Rigsby, Cho and Van Pelt plays with a small ball and then... and then??? Then Ron comes and says to Van Pelt that O'laughlin hangs on the phone. But O'laughlin is Red John's mole. Ron talk to O'laughlin and then puts him through Van Pelt.

ther is more: episode 3x18, min. 09:13: Rigsby and Van Pelt come in the office and found O'laughlin... with Ron on the background.

There were two Red John's disciples and booth were in touch with Ron.
An interesting thing is that Red John needed a mole at the CBI only when his case was in the hands of Bosco. O'laughlin in fact is the FBI mole and when he dies he is replaced (but we don't yet know the new mole). When Rebecca dies, she is not replaced, because the case return to the Lisbon's team. And Ron works in their same office.

But there is more: go back to the 1x10: the beginning of the episode takes place at the University of Leyland. Alex Nelson is invited to go on stage and the audience starts to clap. But wait! Who is in the audience? look at the min. 01:02, down on your right... there is Ron! And this is not a donut shop, this is the "VIII annual neuroscience symposium". And there is also Dr. Miller, who healed Patrick when he was running out. This is very creepy!

So, this is my theory. What do I win? :P :P :P