They will now take on the winner of the remaining quarter-final between Rangers and Partick Thistle.
It may have been the battle of the Saints, but any notions of canonisation went out the stained glass window as both sides made sure there was no quarter spared in an unexpectedly one-sided match.
It was the home side who were the first to test the waters in the eighth minute when Mark Corcoran's corner found the head of Ian Maxwell, but the defender could only watch in anguish as his header flew narrowly over the bar.
Divine inspiration seemed to rain down from the heavens in the 13th minute when on-loan Celtic star, Rocco Quinn, produced the sort of goal that would grace the most hallowed of football matches.
Liam Craig swung in a deep cross and the Perthshire player drilled in a right-footed volley from the edge of the box that swerved wickedly into the top corner, with Chris Smith rooted to the spot.
Calamity struck the St Mirren defence minutes later as the Buddies were too busy trying to claim for offside rather than mark Andrew Jackson and the striker remained calm to tuck the ball into the corner of the net.
The home side resorted to desperate measures midway through the first half when David Barron elected to try his luck from fully 40 yards, but Alain Main watched the shot all the way into his arms.
The heavens were collapsing for the Buddies as on the half-hour mark, St Johnstone made it three. Peter MacDonald seemed to stumble over the ball, evading St Mirren defenders in the process, before picking his spot in the bottom corner of the net.
Gus McPherson elected the bring on Stewart Kean at half-time to freshen up what had previously been an ineffective attacking force, but it was Derek McInnes' men who continued to threaten.
The ageless Andy Millen clumsily hacked down Jody Morris just after the restart and referee Dougie MacDonald had not hesitation pointing the spot.
The normally reliable left foot of Paul Sheerin was found to be fallible as Smith dived to his left for parry the former Aberdeen player's penalty.
Sheerin's evening went from bad to worse in the 70th minute as the defender was adjudged to have handled in the area and Billy Mehmet stepped up to send Main the wrong way from the penalty spot.
St Mirren began to fight back and Maxwell was unlucky to see his header clip the crossbar with ten minutes remaining, as the home side made a contest of the remaining minutes.