As title says, my flight would go directly from Mexico City to Canada without stopping in the US.
But with all the security that US has, I'm not sure if I still need a U.S. Visa because after all, the plane will be flying through United States airspace.
No, you do not need a US visa. You only need a US visa if you intend to stop or change planes in the US. (This visa-free overflight is the same for any other country. For example, I've flown over Russia on my way from Tokyo to Paris but I did not need to obtain a Russian visa.)
In the event that the plane needs to make an emergency landing in the US, there are procedures to handle and process the passengers even if they would normally require a visa to enter the US.
In addition to the information from the other answers, to overfly the US, the airline will have to pass your information to the US CBP according to Secure Flight.
Since November 2010, Secure Flight has conducted watchlist matching of passenger information against the TSDB for all covered U.S. and foreign flights into, out of, and within the United States, including point-to-point international flights operated by U.S. airlines. Secure Flight also performs watch list matching for flights that overfly, but do not land in, the continental United States.
As long as you are not (or someone named the same as you is not) on any of the TSA no-fly or other watch lists, this shouldn't affect you.
Keep the following in mind: never, anywhere in the world, do you need a visa simply for flying over airspace. I repeat, nowhere in the world.
You do need to provide extra information to the airline.
you did need a travel visa if flying over the US airspace. it happened to my wife, when she tried to fly from toronto to vancouver...
they did not allow her to board the place because she did not have a US travel visa, in case the plane had to make an emergency landing int he US
