US citizens traveling to Mexico are not required to have a visa in addition to a passport for entry. Instead, you need a Mexico Visitor’s Permit. This form should be handed out during flights to Mexico or you can get one at a border crossing if entering by road.
During a recent trip to Mexico, one of us (we’re not saying who) lost my Tourist Card (FMM). The discovery was made the morning of departure from Mexico City. I decided to head to the airport knowing nothing of the procedure other than it would most likely incur a fine. At the airline’s check-in counter, I was told to head downstairs to the Immigration office. The Immigration office is located on the first floor of Terminal 1, the same floor as international arrivals. The office is open 24/7. I quickly learned that the fine was 390 MXN pesos (as of February 2016) and there was an ATM conveniently located just outside the office. The whole process took less than 15 minutes. This was before 7 am on a Sunday morning so I’m not sure how the office is during peak times, if there are peak times.
If you find yourself in a similar situation, don’t fret. Just plan to give yourself extra time at the airport on your day of departure and take pesos. US currency and credit cards are not acceptable forms of payment.