I live in Houston and TexMex is, of course, ubiquitous here in Texas. Of course, it's not really Mexican as in, authentic Mexican, but it's authentic TexMex. And as someone said about Mexican food being several different styles of food, well, in fact, so is TexMex. We have several "favorite" TexMex places which all serve somewhat different styles of food, all of which fall into the category of TexMex.
My son lived in Manhattan for a few years and according to him there's no good TexMex there. Apparently there were multiple spots which attempted it, but they didn't quite get there.
As far as authentic Mexican food, yeah, you can get that here too. And it's pretty good. But not as good as the TexMex.
If you need a recommendation for good TexMex in Houston, I've got a list! As a side note, the #1 sign that a restaurant is not athentic TexMex: No free chips & salsa.