Senior Daniel Freshley drilled a bases loaded single into right center with one out in the bottom of the 12th to send the Hornets (26-7-1) past Macon (24-8).
Freshley's walkoff hit, the second of the day for SU, plated Joe Coleman from third.
The Yellow Jackets then elected to intentionally walk Michael Paul, setting the stage for Freshley to get the game-winning RBI.
Freshley laced his single into deep right center, allowing Coleman to trot home with the Hornets second walkoff win of the day.
The win over the 'Jackets also gives Shenandoah the day off Saturday and sends the squad into a 1 p.m. Sunday championship versus the remaining team in the tournament.
Roanoke, Virginia Wesleyan and Randolph-Macon will all battle beginning Saturday morning at 9 a.m. for the right to face SU – Roanoke faces VWC in that 9 a.m. game with the winner taking on Macon at 12:30.
The winner of that contest will have to beat Shenandoah twice to claim the 2015 title.
Coach Kevin Anderson's Hornets rallied from two one-run deficits in the game, including a Wilt two-out, RBI single in the 10th.
Mossman scattered eight hits and allowed two runs (one earned) and three walks against seven strikeouts.
Shenandoah starting pitchers Michael Scimanico and Mossman pitched 18 1/3 of a possible 21 innings Saturday and allowed just two earned runs.