As of February 2025, adults AND children should be prepared to show their passport book/card when ENTERING Mexico. You'll need the number to apply for the FMM (please see more FMM info below).  This may not last, but for now, it's being enforced.   ​A̶l̶s̶o̶ ̶(̶o̶f̶f̶i̶c̶i̶a̶l̶l̶y̶)̶,̶ ̶M̶e̶x̶i̶c̶a̶n̶ ̶b̶o̶r̶d̶e̶r̶ ̶p̶a̶t̶r̶o̶l̶ ̶i̶s̶ ̶s̶u̶p̶p̶o̶s̶e̶d̶ ̶t̶o̶ ̶r̶e̶q̶u̶i̶r̶e̶ ̶y̶o̶u̶ ̶t̶o̶ ̶s̶h̶o̶w̶ ̶y̶o̶u̶r̶ ̶p̶a̶s̶s̶p̶o̶r̶t̶ ̶a̶n̶d̶ ̶F̶M̶M̶ ̶g̶o̶i̶n̶g̶ ̶I̶N̶T̶O̶ ̶M̶e̶x̶i̶c̶o̶.̶ ̶ ̶T̶h̶a̶t̶'̶s̶ ̶a̶l̶s̶o̶ ̶n̶o̶t̶ ̶e̶n̶f̶o̶r̶c̶e̶d̶ ̶a̶t̶ ̶S̶o̶n̶o̶y̶t̶a̶ ̶a̶t̶ ̶t̶h̶i̶s̶ ̶t̶i̶m̶e̶.̶

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YES.  According to all government websites, all travelers 16 and over require passports (books or cards) at all land border crossings back into the United States.  Homeland Security requires proof of identity AND citizenship to comply with the WHTI which is part of the Terrorism Prevention Act created by the 9/11 Commission.  This has become a consistent requirement as of February 2025.  However, citizens will never be denied entry to the US.

Without a passport book/card, the US CBP can legally detain you while they "verify your citizenship".  If they decide to delay you, it would be a bummer of an ending to a fun trip.   Illegal immigration is a hot topic, and the border agents can be lenient one day and follow every rule to the letter the next. If you want to avoid the possibility of being detained when coming back to the US for being "non-compliant", get your passport book/card.  U̶n̶o̶f̶f̶i̶c̶i̶a̶l̶l̶y̶,̶ ̶m̶a̶n̶y̶ ̶v̶i̶s̶i̶t̶o̶r̶s̶ ̶s̶a̶y̶ ̶i̶t̶'̶s̶ ̶n̶o̶t̶ ̶a̶ ̶p̶r̶o̶b̶l̶e̶m̶ ̶w̶i̶t̶h̶ ̶a̶ ̶D̶r̶i̶v̶e̶r̶'̶s̶ ̶L̶i̶c̶e̶n̶s̶e̶ ̶a̶n̶d̶ ̶b̶i̶r̶t̶h̶ ̶c̶e̶r̶t̶i̶f̶i̶c̶a̶t̶e̶ ̶t̶o̶ ̶c̶r̶o̶s̶s̶ ̶b̶a̶c̶k̶ ̶i̶n̶t̶o̶ ̶t̶h̶e̶ ̶U̶S̶.̶ ̶ ̶A̶t̶ ̶t̶h̶i̶s̶ ̶t̶i̶m̶e̶ ̶(̶A̶p̶r̶i̶l̶ ̶2̶0̶2̶3̶)̶,̶ ̶t̶h̶e̶ ̶L̶u̶k̶e̶v̶i̶l̶l̶e̶ ̶C̶B̶P̶ ̶h̶a̶s̶ ̶n̶o̶t̶ ̶b̶e̶e̶n̶ ̶c̶o̶n̶s̶i̶s̶t̶e̶n̶t̶l̶y̶ ̶e̶n̶f̶o̶r̶c̶i̶n̶g̶ ̶t̶h̶e̶ ̶l̶a̶w̶.̶ ̶ ̶H̶o̶w̶e̶v̶e̶r̶,̶ ̶t̶h̶a̶t̶ ̶c̶o̶u̶l̶d̶ ̶c̶h̶a̶n̶g̶e̶ ̶a̶t̶ ̶a̶n̶y̶ ̶m̶o̶m̶e̶n̶t̶.̶