The Spanish government in Mexico first sent explorers, then missionaries and settlers into this area in the late 1700s. Mission Dolores in San Francisco was founded in 1776. They soon found the ground in San Francisco inhospitable for planting, and so by 1815 they established Rancho San Pablo on the opposite coast (in Spanish the contra costa) of the bay to raise cattle and food for the mission. Rancho San Pablo was the first permanent non-Indian settlement in all of what is now Contra Costa county.

The name San Pablo was given to the land in 1811 by Ramón Abella, a priest from Mission Dolores who explored the bay and the northern rivers. He named the two points opposite each other on the bay "Point San Pablo" and "Point San Pedro." (Thus Saints Paul and Peter guarded the bay.)

Mexico gained independence from Spain, and began to divide up land owned by the missions. The overseer of Rancho San Pablo requested permission to claim the rancho as his own and in 1823, Francisco Castro was granted 17,000 acres of land.

This covered land that is now Richmond, El Sobrante, and Pinole, and extended all the way out to the bay. That's why the upper bay is called San Pablo Bay today.