I have a flight from Denver to Mexico with a layover in huston, TX. My question is, do we need to present a negative covid test? I know we need one when we come back from Mexico but do we need one to board on our way to huston?

Correct, COVID tests usually are only required coming into the U.S. You should be fine without one flying to Mexico :)

I'd check with flights out of the ountry.

No only out of the country

No covid test required

I didn't need one flying from Vegas to Denver then to Nashville.