Saints striker Jonah Ayunga ensured his new side weren't the victims of a cup upset after producing a clinical first half brace at Central Park.
The summer signing bagged his first goal for the club early on against Cowdenbeath, turning cleverly past two defenders before firing into the bottom corner to break the deadlock.
And he doubled St Mirren's lead shortly after when he tapped home from Joe Shaughnessy's flick on from a corner.
