Is 54 minutes enough to transfer from domestic to international in Houston?

Yes, I did this successfully at IAH on United with a 30 minute domestic-to-international connection in 2011 and a 32 minute connection in 2012. Tips:

• IAH is big. Study the airport map ahead of time, then check your phone as soon as you land to find out which gate you'll have to get from and to.

• Walk fast. If you're just going one terminal over, like C to E, you can probably walk faster than the airport train.

• Make sure your plans are flexible in case you miss the connection. If United sold you the connection as one ticket and you miss it because the first flight is delayed, United should put you onto the next flight instead.

There are a few important things to consider here:

(1) Historical flight times and timeliness. Using FlightAware, you can see the historic arrival and departure times of both flights. Check out these stats for the past 1-2 weeks. Is the flight arriving to IAH typically on time or typically late? If late, what's the average delay?

(2) Your personal comfort with rushing. I've lived abroad for several years and made dozens of international flights. There's nothing like the anxiety of landing at one airport, knowing you have to fight your way off of a big plane and run across the airport to make the next flight. Are you comfortable trying to run around/ahead of people when deplaning or changing terminals? Are you generally a fast speed walker? Do you have any kind of flight/travel anxiety?

(3) Closing the doors vs. takeoff time. Is the 54-minute transfer window between the landing time of your first flight and the takeoff time of your second flight? International flights often begin boarding 45-60 minutes in advance and often close cabin doors 10-15 minutes before takeoff. This means that if your first flight is even 15 minutes late, you'd have 39 minutes left. It will take:

• 5-10 minutes to deplane depending on where you sit
• 10-20 minutes to transfer terminals
• 5 minutes to get to the Mexican flight departure gate

which leaves as much as 19 minutes (39-5-10-5) or as little as 4 minutes (39-10-20-5) before the flight leaves. If the doors close 10 minutes prior to departure, you will need to be at the very front of the plane and/or know that the transfer takes 15 minutes or less in order to make it by a very close margin.

Personally I always book international flight transfers with 2-4 hours layover. I find it's too expensive/stressful/tiring to rush through the airport for the second flight.

As I usually look for "shortest flight" when booking international travel, I know a lot of the flights will give you transfer times of an hour down to even 40 minutes or less even at the big airports (ATL, JFK, LAX). The airline has much more experience of judging whether passengers make a connection like that and will impose rules on the tickets as they do not want to have to re-book you, so if that was their allowed time then it should be fine with an on-time arrival, providing you are prepared to go straight from aircraft A to aircraft B and as @BethDevine says are okay with setting a reasonable pace.

The second part of the same airline providing both flights of the ticket is that they know you are making that transfer. The gate agents at your international flight are aware there are people on your first flight (and chances are it won't be just you) and will be apprised if it is a few minutes late. I have had a couple of cases of diving across a large airport, kicking old people and children out of my way as I barge my way across, only to find the flight is being held for other people coming from my original delayed flight.

Of course there is no guarantee, if your original flight is delayed an hour or two there is no way they can hold the flight for you but in that case again the airline already knows that. In at least one case Delta had already re-booked me onto the next flight before we even landed at the transfer airport. As soon as I could turn my cellphone on there was a text message with the new flight details.

I tried to find the official Minimum Connection Time for your target airport.

A user on FlyerTalk asserts

I had heard somewhere that the official minimum connecting time in IAH is 25 minutes.

Official MCT is a good indicator in order to make an estimate. Unfortunately the above is not official news but suggests that with 54 minutes you have plenties of time to walk the gate.

Consider that MCT (being a minimum) is normally computed as the maximum time a normal passenger can take to between the edges of the airport.

You have more than double the time, as soon as your flight is on schedule. You are ticketed through, I don't know if that is the same as happens in Europe but the airline should at minimum guarantee you board your connection in case of a delay, by either delaying the connection or offer you compensation.