Mind you is an expression used to say that you should pay attention to the surrounding info, because it either explains or contradicts the rest of the sentence.
When someone says "mind you," what they really mean is something like "bear in mind that..." It can be used to emphasize your point: "He only brought a bag of chips to the potluck, and mind you, he's a gourmet chef!" Or it can be used to mark something that softens your meaning a little bit: "She's not really my favorite person. Mind you, though, I don't know her very well."