Sweet also went the distance in the circle for the Mounties, striking out a pair and not allowing an earned run.
Emma Newberry went 3-for-4 with a double and three RBIs, and Kelsey MacHaffie was 2-for-3 in a 13-hit attack for the Mounties.
Mount Greylock broke the game open with a seven-run third inning to take an 8-0 lead.
Taconic got a 3-for-4 day at the plate from Ella Stodden. Maddie Warrington was 2-for-4 with an RBI.
Rylee Paronto took the loss in the circle, striking out seven in a complete-game performance.
The Thunder (13-7) learns Wednesday who it will face in the first round of the state tournament.
Mount Greylock (14-6) moves on to face top-seeded Greenfield in the regional final on a date to be announced.
The Green Wave beat its first two tournament opponents by a combined score of 22-0.