Originally Posted by
MSPeconomist
ADDED. There was a time when driving 50 miles into Mexico was permitted without a passport. I don't know whether this was for all USA citizens or just those who were native born.
The requirement for travel beyond the free zone isn't for a passport, it's for a tourist visa or FMT issued by the Mexican Government. (though, it's been a while, this might have changed)
In the 90's I flew to Mexico City several times with nothing but my DL and birth certificate and my FMT was issued when I arrived. I don't know if Mexico requires a passport now, I just use it because I'll need it coming home anyway.
You can still walk into Mexico with basically nothing, it's getting back that requires a passport or passport card.