St Mirren beat Motherwell 4-1 in the first of the Premier Sports Cup semi-finals in front of a crowd of 21,277 on Saturday evening.
Mikael Mandron fired Saints in front following a corner in 25 minutes and Dan Nlundulu thumped in a superb second goal from the edge of the box before the break.
Callum Hendry pulled one back with seven minutes left to give the Steelmen hope but Richard King restored the Buddies' two-goal advantage.
Mandron rounded off an emphatic win for Stephen Robinson's side, who will face Celtic or Rangers in the final on Sunday December 14.