Fleck was one of three enforced changes from the side who earned a draw in Bucharest three days earlier, with Kyle Lafferty and Kevin Thomson dropping out of the squad and Kenny Miller to the bench after the birth of his daughter on Friday morning.
Kris Boyd took his place in attack, with Nacho Novo replacing Lafferty on the left of midfield.
It was an attacking set up from Smith and with Rangers seeking only their second home win in six matches the manager couldn’t have hoped for a better start.
After an impeccably observed minute’s silence to mark Remembrance weekend, the home side came out of the blocks with devastating urgency.
Steven Naismith, playing on the right of a four man midfield, made good progress down the right and set up Fleck for a curling left foot shot from the edge of the box.
Saints keeper Paul Gallacher pushed the ball away, but only as far as Novo and although the Spaniard had little time to react, the ball deflected to Boyd who made no mistake, rifling home after just 16 seconds.
Having lost a lead in the dying moments in Romania in midweek, Rangers were clearly keen to increase their lead and continued to push forward in the early stages.
Naismith chased down a loose pass and almost beat Gallacher to the ball, with the keeper doing just enough to nick it off the forward’s toes.
Rangers weren’t to have it all their own way though and Allan McGregor was called in to action in the tenth minute, when he pushed Andy Dorman’s long range effort round his post for a corner.
And the Gers goalkeeper was grateful to see Garry Brady’s curling effort crash back off his post twelve minutes later when the midfielder beat McGregor from the corner of the Gers box.
The home side might already have been further ahead at that stage with Fleck and Boyd again almost combining to double the advantage.
The 18-year-old again tested Gallacher from distance and with Boyd closing in on the rebound he would have been disappointed to see a left foot effort sliced wide of the post.
Fleck was a constant thorn in the Saints’ side and when he was presented with the chance to deliver from a corner after Steven Whittaker’s forward run had forced the set piece he did not disappoint.
His inswinging ball was flicked on and the ball should have found the net from Lee McCulloch’s right boot, but the midfielder blazed high and wide with 10 minutes of the half remaining.
That lung bursting run was the only time Whittaker had been seen in his usual attacking positions and he wouldn’t be back out for the second half.
The Scotland international had complained of tightness in his hamstring after the exertions on a heavy pitch in Romania in midweek and he made way for DaMarcus Beasley.
That saw Steven Davis move to right back behind Beasley, and it was the Northern Ireland man who almost set up Boyd for his second.
A driven crossed ball was chased down by the striker, but he was well marshalled by his marker who forced the ball over the top of the bar for a corner.
And from that set piece Boyd would eventually grab his second when St Mirren failed to clear properly and the home side worked the ball through to Novo.
The little forward, looking to make amends for his part in Unirea Urziceni’s goal three days earlier fired in a shot, but he dragged it across goal where Boyd was lurking to give Rangers a two goal cushion.
And moments later he almost completed a hat trick when he got his head to Davis’ cross, but could only nod the ball on to the top of the bar, under pressure from the visitors’ defence.
The Gers number nine was hungry for another treble and was presented with another fine chance after great work from Naismith released him in the box.
Naismith burst from the half way line past a couple of Saints defenders and played a lovely reverse pass in to the path of Boyd, but the striker’s left foot drive though well hit, was equally well saved by Gallacher.
Gallacher should have been given no hope of preventing a third with ten minutes remaining when Novo and Beasley sprang the offside trap.
However Novo’s attempt to pick out the American in the middle was poor and when the US international did gather, his attempt at goal was just as disappointing.
In the aftermath of that attack Novo signalled to the Rangers bench that his calf was causing him a problem and he was soon replaced by Kenny Miller.
Miller and his team mates were made to endure a nervous final few minutes when, just as they had done in midweek, Rangers conceded with just two minutes remaining.
Davie Weir was furious with the officials in the aftermath of Stephen O’Donnell’s consolation strike, but the Saints midfielder was left unmarked to turn and fire beyond McGregor.
However despite some obvious signs of nerves Rangers held on to secure the three points that took them back to second spot in the SPL.