The Mentalist was a more serious version of Monk. I think that the connection between Psych and The Mentalist is tenuous (despite the references in Psych). As for the connection between Monk and The Mentalist, here are the character comparisons…
Patrick = Monk (the main man loses his wife to a killer who takes seasons to be caught*).
Cho = Leyland (the straight-faced detective).
Rigsby = Randy (the dim-witted sidekick).
Lisbon = Natalie (the main assistant).
Van Pelt = Sharona (the lesser assistant who eventually dates the dim-witted sidekick).
Both TV shows have an episode involving a parachute blunder. Both TV shows have an episode where the detective is blind. I won’t tell you which episodes because it would spoil the surprise. Still, fans of The Mentalist can hardly say that the blind connection was a coincidence. As for the M connection, these TV shows are the ultimate murder mysteries whose content is embodied by the M in the titles.
The makers of Psych seem to think that The Mentalist is an imitation. However, The Mentalist was about someone who previously pretended to be psychic. To the credit of those behind Psych, The Mentalist had an episode about an Old West tourist attraction. Psych and Monk have episodes about murdered food critics. As for other similarities, there are episodes about retirement homes, fake aliens, daredevils and murdered models.
* Intriguingly, this is the same premise as Life and Endgame.