These count more as Picard's mind being "messed with", not getting into his mind so much as subtly altering his perception of reality to make him do things for specific purposes:
Where Silence Has Lease - an entity (Nagilum) plays mind games with Picard (and in fact the entire crew) and gets in his head to the point where Picard won't cancel the auto-destruct until the last second in case the Enterprise is really still in a void. All so the entity can provoke Picard to learn more about humanity.
Allegiance - Picard is kidnapped and held prisoner, apparently along with three others, when in fact he is the only prisoner, in an attempt by aliens to learn about authority and leadership.
Actually, I could (partially tongue in cheek) argue that in many if not most episodes, starting at Encounter at Farpoint (where Q played the ultimate mind game with Picard, putting him on trial and trying to make him admit to the crimes of humanity) to the final episode where Q once again messed with Picard's mind to try to get him to solve a temporal puzzle involved some kind of attempt to take over, control, or influence Picard's mind, and as a logical extension of that, his ship, his crew, or some combination of the above.