Founded by Junipero Serra on July 14, 1771.

3rd Mission of California. And yet, it is so isolated, that it is the least visited Mission these days due its location inside of a (inactive) military installation, according to the local National Park Ranger.

San Antonio is located at the edge of military installation of Fort Hunter Liggett in the small town (or village) of Jolon, California. The address is: 1 Mission Creek Rd, Fort Hunter Liggett, CA 93928. Even its official web page names it "The Mission which the Time Forgot". Visitors can find it while driving the Highway 101 from Salinas toward Paso Robles and San Luis Obispo. Use exit 252 from Hwy 101 (north or south) and travel 26 miles (!) on Jolon Road to find the entrance to Hunter Liggett, close to the Dutton Hotel Ruin and the St. Lukes Episcopal Church, a tiny wooden building. Follow the road for 11 more miles (!!) to find the Mission. The road itself is so underdeveloped and rural that after passing the premises opposite the IHG Army Hotels Gibb Hall at the Fort Hunter I took the wrong fork leading into the wilderness, and had to turn back. I found this Mission itself quite impressive and fascinating. Its museum is definitely worth the extra drive from the Highway 101.
Wikipedia page.