Can one walk into Mexico from the US with a US green card and without a passport?

Officially foreigners (ie not Mexican citizens, nationals, or legal residents) are required to present a passport with them when entering and carry it while in Mexico. The Mexico embassy website states that a valid travel document (ie Passport or refugee document etc - something that gives you the ability to travel to foreign countries) is required to enter Mexico.

See https://consulmex.sre.gob.mx/sanfrancisco/index.php/servicios-para-extranjeros which says

• All foreigners, regardless their nationality, are required to present a valid and not expired passport or travel document when entering Mexico (traveling by air, land or sea).
• The Mexican government only requires that your passport must be valid during the entirety of your trip. Please contact the airline(s) you will be flying on to ensure that they let you board the plane as the United States does have a six months validity rule for passports that may apply.

In practice you can drive over the border and will never be asked to show a passport. Walking over it's probably 50/50 whether you get asked or not. And I believe in the current covid time they are asking everyone walking over to show a passport.

When entering you are supposed to get an FMM travel permit, which requires you to present your passport in order to obtain the permit, but a lot of the time it is up to the traveler to do this, you aren't always stopped.

I do know several American citizens who live in Mexico and do not have (have never had) a passport and have no other legal residency standing in Mexico. They have and do cross the border frequently without issue in either direction.

My examples are for the San Diego/Tijuana border crossings - ports of entry in other areas may be easier or more difficult.

The answer depends if you're planning to return to the USA or not. It also depends on where you intend to cross. Different bridges have different rules and processes in place.

In my 40 years of experience of crossing into Mexico on foot at the Progreso International Bridge in Texas, I have never once been asked for identification going into Mexico. In fact, last September I walked across (with a large group of hunters) during Covid and re-entered the USA with my Texas drivers license. I usually try to bring my passport but sometimes forget it. A drivers license will work, or you can even use no identification to re-enter, but you will wait for a short interview and some questions. I have done this before after many margaritas.

A green card would not work for entering Mexico, because Mexico does not issue green cards or even use them. A green card is a US form of identification, which holds no bearing in Mexico.

If you intend to travel into Mexico, beyond the border crossing towns and bridges, you will need a passport. A green card will not work with Mexican authorities in the interior parts of Mexico.

If you intend to travel into Mexico, for the day (for shopping, food, medicine, etc) and walk back to the USA, you can use your green card to re-enter the USA. USCIS can easily identify you with it. I hope that helps.