The match was slow to start with, a shot from Saints' Andrew Stirling in the 7th minute, which went high over the bar from 20 yards being the only chance early on.
Rory Loy was starting to threaten the Buddies down his left side with some dazzling play and it was the Internationalists cross on 13 minutes which saw the in running Andrew Shinnie head into the roof of the net from 6 yards and put the Light Blues one up.
The ex-Killie youngster was tearing the home side apart down the left flank and had a chance to double the visitors lead on 23 minutes but Arturs Vaiculis saved well from 8 yards.
Loy and Shinie were to link up again on 32 minutes when another cross from Loy was met by Shinnie on the edge of the 6 yard box but this time the shot went wide.
By this point St. Mirren were on the rack, but to their credit were holding out and Stirling came in from the left to hold off Alan Lowing on the edge of the box to unleash a more accurate shot than the last time only to see Scott Gallacher save well.
Victory Shield hero, John Fleck, had a great opportunity to add his name to the scoresheet on the 38th minute but his first touch let him down when played through into the box, the young Scotland star regained control of the ball and tried a deft chip against the on rushing Vaiculis, only for the Goalkeepers height to prove advantageous to the home side as he collected the ball in mid flight.
Half Time St. Mirren 0 Rangers 1
The home side came out for the second half with a more positive game plan and Centre Half Marc McAusland came within an inch of equalising when his header from a free kick went just over the bar.
The young Buddies were certainly fired up and after a few meaty challenges Referee Kevin Clancy lost patience with the warnings and booked Calum McGregor for a late challenge on John Fleck.
The change in the home sides play got them their reward on 55 minutes when Stephen McGinn equalised from 4 yards when a long range shot rebounded off the cross bar straight to the midfielder.
Andy Shinnie tried to fire Rangers back in front on 61 minutes when through one on one but again Vaiculis came to the rescue for the home side.
Steven Lennon was to then try his luck on 67 minutes with a shot from 30 yards which had Vaiculis scrambling to push the ball round for a corner kick and from that corner Lennon again tried to find a breakthrough but again the Buddies keeper thwarted him.
Although Rangers were now pushing forward more Craig Marr almost gave the home side the lead on 76 minutes when his flashing hearer went narrowly past.
There were certainly no prisoners being taken in a match both sides clearly wanted to win and Saints goal heron Marc McAusland was carried off the pitch before Alan Lowing was next to find his name in the Ref's notebook following a late tackle on Stirling.
As the match entered the final stages, with extra time looming, both sides went for broke as firstly Mark Docherty saw his header touched over by Gallacher before Billy Kirkwood played his trump card and brought on Craig McShea for the exhausted Rory Loy. McShea's first touch was a rasping shot from the edge of the box which Vaiculis held well, following that the Under 17 starlet tore down the right flank and just before the byline unleashed a driving shot which was goalbound before being cleared off the line, however the clearance then went straight to Dean Furman who was being closed down by Vaiculis and the ball ricocheted into the net with 2 minutes left and the ex Chelsea youth player claiming the goal which won the match and put the Light Blues into the Semi Final.
The Teams
St. Mirren - Vaiculis, McGregor, Carr, Docherty, McAusland, Gaffney, MacKay, McGinn, Marr, Weir, Stirling,
Subs - McShane, Adam, McMenamin, Rawcliffe, McColgan
Rangers - Gallacher, Lowing, Kinniburgh (McLachlan 67), McMillan, Perry, Emslie (Djeziri 59), Loy (McShea 86), Shinnie, Lennon, Furman, Fleck
Subs Not Used - Sagar, Harvey