The Light Blues were ahead inside the opening 15 minutes, before Alex Gogic pulled one back for the visitors in the first-half. The decisive winning strike came on 70 minutes, as Cerny netted his third goal in two games to secure all three points in the Scottish Premiership.
There were four changes from Thursday night’s 4-0 victory over FCSB in the Europa League as Tom Lawrence and John Souttar missed out through injury, with Mohamed Diomande and Robin Propper coming in in their place.
Neraysho Kasanwirjo made his first league start for the club, while Clement opted for Hamza Igamane from the start, with James Tavernier and Cyriel Dessers on the bench.
That meant Rangers lined-up with Jack Butland in goal, behind a back four of Kasanwirjo, Propper, Leon Balogun and Jefte.
Connor Barron and Nico Raskin retained their places in midfield and were joined there by Diomande, while Nedim Bajrami and Vaclav Cerny provided the support for Igamane.
The first sight of goal came for the visitors, as Killian Phillips lifted a ball over the top and into the path of Toyosi Olusanya. The St Mirren forward took a touch into the box before releasing a right-footed shot across goal from a tight angle but it flashed just wide of Butland’s post.
With 13 minutes on the clock, it was Rangers who had the opener at Ibrox as Mohamed Diomande scored his first goal of the season.
It was a lovely move down the left, with Nedim Bajrami showing patience to pick-out Nico Raskin in space on the edge of the box and the midfielder blasted a powerful strike towards goal.
St Mirren goalkeeper Ellery Balcombe blocked the shot, but Diomande was alert to the rebound to guide a composed left-footed finish into the back of the net.
However, with 25 minutes played, the visitors were back level from a corner, as Alex Gogic headed beyond Butland from a Mark O’Hara delivery. The goal was checked by VAR for a possible foul on the Rangers number one, but after review the header stood to draw the sides level at Ibrox.
Clement’s men looked for a quick response, as they broke forward with a nice move as Bajrami, Diomande and Barron combined to switch the ball out to the right. Cerny’s pass to the overlapping Kasanwirjo was blocked, but the right-back managed to retrieve the ball and hit the bar with his effort.
Bajrami was next to have a shot of his own as he got on the end of a Kasanwirjo cross at the back post but could only hit the side netting with his left-footed attempt.
Cerny, high on confidence from his two goals on Thursday night, tested Balcombe with a low drilled strike as he weaved his way through St Mirren bodies to create an opening at goal, but the visiting goalkeeper was able to gather.
There was a change in attack for Rangers at half-time, as Clement introduced Cyriel Dessers up front in place of Hamza Igamane.
Early in the second half, Olusanya had the ball in the net again for St Mirren as he broke through goal one on one with Butland and finished a low shot beyond the goalkeeper but the linesman raised his flag and the goal was disallowed for offside.
At the other end Bajrami had a strike from distance pushed away by Balcombe, before the visitors were able to clear the danger.
From a free-kick, Balcombe produced a good save to tip over a Cyriel Dessers header after he got on the end of a lovely delivery from Bajrami to head it towards goal.
It was a lively start to the second half, and before the hour mark Butland was forced into another save as he broke away from Robin Propper to get a shot away at goal but the Rangers number one matched the effort.
Captain James Tavernier was introduced with 60 minutes played, as he came on to replace Robin Propper.
With 70 minutes on the clock, Rangers were ahead again as a quick throw in down the left opened up the attack with Cyriel Dessers cleverly clipping the ball over the defenders to drive to the byline.
The striker then pulled the ball back into the path of Vaclav Cerny who slotted a left-footed finish beyond Balcombe to restore Rangers’ advantage.
Cerny was denied a second of the afternoon in the final 10 minutes with Balcombe pulling off a great save in the St Mirren goal.
Diomande and Bajrami combined, before the latter picked-out Cerny with a lovely ball. His strike looked to be going into the back of the net, but the goalkeeper got enough of a touch to tip it just behind the post for a corner.
With six minutes added on at Ibrox, Dessers saw a chance deflected wide for a corner as he got on the end of a ball from Dujon Sterling. The Light Blues were unable to find that third goal in the dying stages but had done enough to secure all three points this afternoon.
RANGERS: Butland, Kasanwirjo, Propper (Tavernier, 62), Balogun, Jefte, Barron (Sterling, 85), Raskin, Diomande, Bajrami, Cerny (McCausland, 88), Igamane (Dessers, 46)
Subs: Kelly, Dowell, Fraser, McKinnon, Lovelace.
ST MIRREN: Balcombe, Tanser, O’Hara (Boyd-Munce, 79), Kiltie (Idowu, 68), Gogic, Scott (McMenamin, 62), Dunne, Olusanya, Fraser, Bwomono (Mandron, 79), Phillips.
Subs: Urminsky, Ayunga, Smyth, Adeniran, Penman.