Mi nacionalidad es mexicana.

Mi without an accent is the possessive adjective meaning my.

• mi nacionalidad = my nationality

Mí with an accent is a pronoun used after prepositions, like for me or to me.

• para mí = for me
• a mí = to me

So in this sentence, mi is correct because it is showing possession.

Spanish uses ser for things that are seen as essential, defining, or permanent characteristics, and nationality normally falls into that category.

• Mi nacionalidad es mexicana. = nationality is a defining fact
• Está would sound wrong here, because estar is generally used for location, condition, or temporary states

So es is used because nationality is treated as an inherent characteristic.

Because mexicana is agreeing with nacionalidad, not with the person.

• nacionalidad is a feminine noun
• adjectives that describe it must also be feminine

So:

• Mi nacionalidad es mexicana.

Even if the speaker is male, the word still stays mexicana, because it matches nacionalidad, not the speaker.

A common clue is the ending -dad, which is almost always feminine in Spanish.

Examples:

• la ciudad
• la universidad
• la nacionalidad

So you say:

• mi nacionalidad mexicana
• not mi nacionalidad mexicano

Also, if you use an article, it would be la nacionalidad.

Yes.

That is because mexicana is describing nacionalidad, which is feminine. It is not directly describing the man.

Compare:

• Soy mexicano. → a man says this
• Soy mexicana. → a woman says this
• Mi nacionalidad es mexicana. → anyone says this, because nacionalidad is feminine

Yes, and that is usually the more natural, everyday way to say it.

• Soy mexicano. = said by a man
• Soy mexicana. = said by a woman

Mi nacionalidad es mexicana is correct, but it sounds a bit more formal or official, like something you might see on a form or in a formal introduction.

In Spanish, nationalities are normally written with lowercase letters.

So Spanish writes:

• mexicana
• estadounidense
• colombiano

This is different from English, where nationality words are capitalized:

• Mexican
• American
• Colombian

So lowercase mexicana is correct.

Because Spanish usually does not use an article before a possessive adjective like mi, tu, or su.

So you say:

• mi nacionalidad
• not la mi nacionalidad

This works like English:

• my nationality
• not the my nationality

Normally, no.

México is the country name, not the nationality.

• México = the country
• mexicana = the nationality adjective
• mexicano / mexicana = also a noun for a Mexican person

So the correct sentence is:

• Mi nacionalidad es mexicana.

If you want to use the country name, you would say something like:

• Soy de México. = I am from Mexico.

Here, mexicana is an adjective.

It is describing nacionalidad:

• nacionalidad mexicana = Mexican nationality

In the sentence Mi nacionalidad es mexicana, the adjective comes after es because it is functioning as the complement of the sentence.

Yes. That is also correct.

• Tengo nacionalidad mexicana = I have Mexican nationality

This sounds a little formal or administrative, similar to Mi nacionalidad es mexicana.

In casual conversation, Soy mexicano / Soy mexicana is usually more common.

Spanish usually prefers a nationality adjective such as mexicana, peruana, chileno, argentina, etc.

So instead of building the idea with something like of Mexico in this sentence, Spanish naturally says:

• Mi nacionalidad es mexicana

This is simply the normal Spanish pattern: noun + ser + nationality adjective.