You may find that the name of the place from which your ancestor came has changed or that Mexico’s states or boundaries have changed. The boundaries of Mexico have changed and expanded since the beginning of the colonial period (1519–1821). During the first decade after the capture of Mexico City, the Spaniards generally confined their expansion to the regions south and west of the capital. Throughout the next three centuries the Spaniards of New Spain eagerly pushed the borders northward in response to French and English settlers.

In 1836, Texas declared its independence from Mexico. After 10 years of difficult independence, Texas joined the United States. A subsequent war with Mexico finalized the incorporation of Texas into the United States. In the 1848 Treaty of Guadalupe Hidalgo, Mexico ceded the northern third of its territory to the United States. This land eventually became New Mexico, Arizona, Colorado, Utah, and California. The Rio Grande River was established as the northern border of Mexico by the 1853 Gadsden Purchase, which transferred additional Mexican territory to the Arizona and New Mexico areas.

You may need to determine previous boundaries and jurisdictions to locate your ancestor’s records. Gazetteers and histories can help explain these changes.

The following books explain more about Mexico’s historical geography. You can find these and similar material at the Family History Library and many other research libraries.