The Spanish soldiers were stationed at the barracks to guard the Mission San Juan Bautista founded on June 24, 1797, by Fermín Lasuén.

The Mexican secularization act took most of the mission's lands and buildings way and gave them to connected people.

At its peak, seven stage lines routes passed and stopped in San Juan, with 11 stagecoaches departing daily.

There were stagecoach lines that were heading to San Francisco, Los Angeles, Monterey, Watsonville and Santa Cruz and Salinas.

Plaza Hotel also supported travelers to the New Idria Mercury Mine southeast of the town, opened in 1854 and closed in 1972.