Final answer:
The Spanish built Mission San Diego de Alcalá first as part of their efforts to establish a Spanish presence in Alta California.
Step-by-step explanation:
The first mission built by the Spanish was Mission San Diego de Alcalá. It was founded on July 16, 1769, and served as the headquarters of the Alta California missions. The purpose of the mission was to convert the local Native Americans to Christianity and establish a Spanish presence in the region.