In the early 1700s, Spanish missionaries moved north from Mexico into California and needed wine for religious ceremonies.  They originally transported Mexican wine for this purpose.  But accidents and difficulties along the route led them to plant their own vineyards.  The cuttings they used were brought from Mexico and of Spanish origin.  In California this grape variety was called simply Mission.  And for the next fifty years, all of the wine made in California was 100% Mission grape.

Source: The Wine Bible by Karen MacNeil