U.S. citizens do not need a passport to enter Mexico by land or sea. Mexican border officials 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. Every day hundreds of people cross from Tijuana to San Diego with only a driver's license ... some with nothing at all.
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 linkis.
Yes, you need a valid passport for a flight to Mexico.
Yes, babies need a passport to travel to Mexico.
Yes, children need a passport to travel to Mexico.
Yes, infants need a passport to travel to Mexico.
Yes, children need a passport to fly to Mexico.
Yes, children need a passport to travel to Mexico.
Yes, minors need a passport to fly to Mexico.
Yes, minors need a passport to travel to Mexico.
Yes, you need a Mexican passport to travel to Mexico.
Yes, you do not need a passport to fly within Mexico if you are a Mexican citizen. However, if you are a foreigner, you will need a valid passport to travel within Mexico.
Yes, you need a valid passport to travel to Mazatlan, Mexico.
Your passport