US citizen born in Mexico

As has been noted, those born in Mexican territory are citizens of Mexico, regardless of the nationality of their parents.

The Mexican nationality law acknowledges that a Mexican by birth may possess another nationality. If that is the case, however, such an individual must always enter and leave the country as a Mexican (by presenting a Mexican proof of citizenship).

Mexico actively encourages dual citizenship (Doble Nacionalidad) and US/Mexican nationals should carry citizenship documentation for both countries when traveling to Mexico.

Timatic, the database used by airlines, lists the what Mexican documents are acceptable if you don't have a Mexican passport:

• national ID card
• certified copy of birth certificate
• consular registration
• certificate of Mexican nationality
• naturalization letter
• Matricula Consular
• Constancia de Identificacion
• voter's certificate with photo