Walter Smith made two changes to his starting eleven with Mo Edu replacing the injured Lee McCulloch in the heart of midfield.
But the biggest surprise was that Nacho Novo started on the bench, with DaMarcus Beasley coming into the first XI as his replacement.
It was to be a painful night for the American who took a bruising challenge to the chest from John Potter after only six minutes.
He was stretchered off after a lengthy treatment but, remarkably, came back on to help Rangers open the scoring on 12 minutes.
The winger played the ball inside to Mendes who found the overlapping Papac with a tremendous pass.
The Bosnian got to the byline and his pin-point cut-back was drilled home by Kris Boyd for his 26th goal of the season.
It was a terrific move but it was not long before Beasley was kneeling on the turf clutching his chest and Nacho Novo was called over by Ally McCoist as his replacement.
With 16 minutes played Rangers should have doubled their advantage with Boyd guilty of missing a snip.
Bougherra launched a deep cross in to the box and when Howard flapped at the ball Novo played it back to Boyd who hammered it wide of the empty goal from six yards.
Rangers’ second was not far away and it came from Mo Edu.
Mendes was the supplier with an in-swinging corner and the athletic American rose majestically to head the ball home.
Rangers continued to dominate the game up to the half-hour mark with the home support voicing their frustration at this point.
But the visitors’ injury woes were to build up on 35 minutes when Papac had to be replaced after taking a sore knock to the ankle from striker Billy Mehmet.
As a result Whittaker moved to the left with Christian Dailly filling the right-back slot.
Gus MacPherson’s men enjoyed a brief period of pressure on Neil Alexander’s goal but, overall, it was impressive first-half showing from the Gers with Edu and Mendes, in particular, running the show.
The second period started with Rangers again on top and four minutes in they had a penalty claim when Davis played a teasing cross in from the right and when Boyd headed goalwards it appeared to come off the arm of a Saints defender.
Ref Eddie Smith waved away any pleas and, to be fair, it may have been harsh on the Saints had it been given.
This did not deter the Gers who kept attacking with intent and Boyd again tested Howard at his near post after he had been found by a first-time pass from Mendes.
At this stage Rangers were cruising but, against the run of play, St. Mirren got a goal back before the hour mark.
A Gers counter attack broke down because of a wayward Mendes pass and when Craig Dargo picked up the ball he slid it through to Dorman.
The attacking midfielder sprinted past Davie Weir and his left-foot shot across Neil Alexander hit the post before rippling the net.
It was a poor goal to concede and one which must have left Walter Smith scratching his head in the stands with the threat of another Hearts-style comeback a distinct possibility.
Gus McPherson threw on man-of-many-clubs Jim McIntyre for the last ten minutes of the game and the striker’s physical presence added to St. Mirren’s threat.
But the Gers’ defence stayed tight in the latter stages and credit must go to Neil Alexander and the club’s backline for holding their nerves.
Getting three points from this fixture was all that matters and, with Celtic beating Falkirk 4-0 at Parkhead, Rangers stay close behind their Old Firm rivals in the title race.
In terms of positives to take from tonight’s game Mo Edu was impressive but Smith will be concerned at his side’s dip in concentration which gave the Saints hope of getting a point and the injuries to Beasley and Papac.