Can I re-enter the US after a visit to Mexico if my VWP admission period expired?

The Department of State website states:

If you are admitted to the United States under the VWP, you may take a short trip to Canada, Mexico, or a nearby island and generally be readmitted to the United States under the VWP for the remainder of the original 90 days granted upon your initial arrival in the United States. Therefore, the length of time of your total stay, including the short trip, must be 90 days or less

In other words, if you drive down to Mexico, say, 80 days after you were admitted to the US, you will only have 10 days left if re-entering the US on the same day, or 9 days if re-entering one day later, as the 90 day-clock continues. The same applies if you go to Canada, Bermuda or the Caribbean.

You would need to go at least to Guatemala to formally restore the clock; however, even if entering the US directly from Mexico, the US official could choose to restore the clock if he/she is satisfied that your visit to Mexico was not a visa run, and especially if it's been over 90 days since your original admission into the US. If, however, the officer checking you on re-entry does assume you're trying to live in the US through visa-runs, you'll be refused entry and never be let in visa-free again.

Again, the 90-day clock continues while you're in Mexico, and once you're done visiting Mexico, it will be well over 90 days since you were admitted into the US. Therefore the CBP official must choose to grant you a new admission period in order for you to be let in, which requires them to be satisfied that the your visit to Mexico was not a visa run, and that you will not establish residence in the US. In this case, they will grant you a new admission period (though not necessarily the full 90 days)

The best thing you can do is go to Guatemala and fly to the US from there. This way, the clock is technically automatically restored, which could make re-entry easier. Nevertheless, have the confirmation of your return flight back home ready to show them you're only transiting the US on your way back home

All-in-all, this itinerary should be doable, but it is somewhat risky.

When exiting the US for Mexico, don't forget to show up at immigration, by parking your car on the Mexican side and walking onto the pedestrian US-bound path (before going back to the Mexican side), to have your exit electronically recorded, which could make re-entry smoother. A Mexican entry stamp will also help with this.

The Visa Waiver Program allows you to stay in the US for up to 90 days. In a general sense VWP does not limit the number of 90 days stays that you can have.

In order to avoid people doing "visa runs" to nearby countries to reset the 90 days, they instituted a policy whereby time spent in such countries (including Canada and Mexico) does not generally reset that 90 days clock.

In the days of paper I-94W forms (the card you used to get when going through immigration, and return when you departed the US) this was implemented by you keeping the I-94W card when departing to Canada/Mexico, and then when you returned to the US you showed it to immigration and in effect your existing stay within the US simply continued, and thus your initial entry data was still used as the start date of your 90 days.

Today I-94W forms are (mostly) gone, but fundamentally it works the same way - just using the records on the computer. When you return to the US, the immigration officer can see your previous entry, and can choose to either extend it (thus keeping your initial entry date to the US) OR they can choose to start a new entry record for you, refreshing your 90 day clock back to zero.

In general, they will do the former - as that's the entire intent of the rule, to avoid things like "visa runs".

However there is one situation where they can't do this, and that's if you are now over 90 days beyond your initial entry. At that point they can't continue your existing I-94W, so they have a choice of either starting a new one, or refusing you entry to the US.

Which of those two options they take will depend entirely on the situation. If it appears to them that your trip to Mexico was done in an effort to extent your stay in the US, then you can likely expect them to refuse entry (especially if this was not the first time you'd done it).

However, if they are of the belief that your trip to Mexico was a bona-fide trip in it's own right, and in particular if you are able to show proof of a confirmed flight out of the US within a (very) short period of time, then they will almost certainly grant you a new stay within the US. In practice, it's likely that this will be a full new 90 day stay, however it's also very possible that they will give you a shorter period - 30 days or possibly even less - on the grounds that you are claiming you will be departing the US well within that time frame.

There is one further complicating factor here if you are planning to cross the boarding into Mexico by land (rather than flying), which is that your exit from the US will likely not be correctly recorded. It is critical that you obtain proof that you did enter Mexico (which thus proves that you left the US), and when you did it - be sure that your passport is stamped by the Mexico immigration staff, and that the date on that stamp is readable. It also wouldn't hurt to hang onto a few things like hotel receipts/etc from Mexico for your first few nights to prove you were there - you almost certainly won't need them, but it never hurts to have too much proof!