Saint Louis sophomore outfielder Patrick Clohisy went 3-4 with a home run, three runs scored and an RBI, but a five-run ninth inning pushed Marshall past the Billikens, 10-5.
The Thundering Herd moved to 3-0 to start the year, while the Billikens dropped to 0-3.
SLU got the scoring started early, pushing across three runs in the opening frame, while Clohisy's solo shot to right-center field made it 4-0 in the second inning.
Marshall plated five runs in the sixth to take a one-score lead, but SLU answered in its half of the frame, tying things up on a Tyler Fogarty fielders choice.
The teams entered the final frame knotted up 5-5, but Marshall posted five runs in the inning to come away with the 10-5 win.
The Billikens hit the road to Fort Myers, Fla., for a three-game series against Minnesota beginning Friday.