Ampudia began his career in the Spanish army, and emigrated to Mexico following the Mexican War of Independence. In 1836, Ampudia served with the Mexican artillery at the Siege of the Alamo and later saw heavy combat at the Battle of San Jacinto. During border skirmishes with Texas in the early 1840s, Ampudia commanded the 350-man garrison of Ciudad Mier which was attacked on December 26, 1842, by Texan militia. In a bloody two-day battle, over 600 Mexicans were killed but they eventually forced the enemy to surrender, earning the grudging respect of the Texans across the border.