From Mexico.usembassy.gov:
As of March 1, 2010, all U.S. citizens - including children -- must present a valid passport, book or card, for travel beyond the "border zone" into the interior of Mexico. Entry by any means, for example by plane or car, is included in this requirement.
The "border zone" is generally defined as an area between 20 to 30 kilometers of the border with the U.S., depending on the location. Stays of less than 12 hours within the border zone do not require a visa or tourist card.
U.S. citizens traveling as tourists beyond the border zone or entering Mexico by air must pay a fee to obtain a tourist card, also known as an FM-T, available from Mexican border crossing points, Mexican tourism offices, airports within the border zone and most airlines serving Mexico.
U.S. citizens do not need a passport to enter Mexico by land or sea. The Mexican officials at the border do not ask for any identification from you. On your return home, the U.S. officials would like you to carry a current passport but they can't obligate you to do so. Hundreds of people cross every day with only a driver's license.
To fly into Mexico on a commercial airline, yes, the carrier will most likely insist on a current passport.
The passport question is controversial and confusing. Because of that, a group of people at the San Diego-Tijuana border put together a FAQ on the subject (see related links).
The passport question has become confused and complex since 9/11. Broadly speaking, U.S. citizens don't need passports while Mexican citizens do.
A group of people who live on the border between the U.S. and Mexico produced a FAQ to address these confusions:
http://realtijuana.blogspot.com/2010/07/do-i-need-passport-to-visit-baja.html
No, but you will need a passport to get back into the US.
Yes you do
its called a passport.
does a citizen of Mexico need a passport to go from the us to Mexico and back? He is legal to be in us but not a us citizen.
No, you don't need anything more than your passport.
Indeed.
Yes, Mexico is a foreign country.
No, they don't. People from most European countries just need their passport to enter Mexico.
He needs to the passport application.
Any kind, as long as it is a valid passport. Don't forget the visa, which has to be processed on the US Embassy (Mexico City).
If entering by land, all you need is an ID or birth certificate. If flying, you need a passport.
Citizens from Trinidad & Tobago don't require a visa to visit Mexico. All they need is a valid passport.