The Mexican National flag was first designed in 1821, shortly after the independance from Spain.

The three colors were to symbolise the three pillars of the newly created Mexican nation, as outlined in the Plan de Iguala. They are: Religion (white), Independence (green) and Union (red).

The Constituent Congress formalized the layout of the flag, stipulating that the strips had to be vertical with the green area closer to the flagpole. In the center of the flag and of the white area should be placed the National Shield, consisting of an eagle of Mexican origin, without crown, standing on a nopal growing out of a rock surrounded by water. The eagle had to be on its left leg, while holding a serpent with the other. In addition it had to be framed by the republican symbols: two branches of oak and laurel.

The National Shield was first modified by Fench puppet emperor Maximilian, to have the eagle placed under the crown of the French empire. Soon, Porfirio Diaz ordered that the eagle was placed front and with its wings extended. Later, the shield was again modified by Venustiano Carranza, with the eagle shown on its left side, and restoring some of the features first portrayed by the Aztecs (mexicas) when they first arrived on the site where Tenochtitlan was to be founded. Some presidents have made changes to the position of the eagle, but generally the national shield remains as it was designed by Venustiano Carranza.

The Mexican citizens owe respect the national flag and its symbolism. It even has its own national holiday on February 24.