Starting pitcher Ryker Fox mowed down A's hitters giving up just one hit and one walk while striking out nine batters in seven innings.
St. Joseph (1-0) provided 3,145 fans -- an opening day record -- with all of its offense in the first inning.
With the bases loaded in the bottom of the inning, Zac Johnson lined a base hit to left scoring Ryan Abernathy.
Brandon Huske followed with a run-scoring fielder's choice to anchor the Mustangs to a 2-0 lead.
The A's cut the deficit in half with a run on a passed ball in the eighth inning, but Brett Ash shut the door with a pair of strike outs in the ninth for the save.
Abernathy and Johnson were the lone Mustangs to put together multi-hit games. St. Joe totaled eight hits on the night, and left nine on base throughout the game.
Game two comes Wednesday as St. Joseph welcomes in the Rossville Rattlers for a second non-league game.