Storm Boelo wreaked havoc on the conditions throughout the 90 minutes, but the Gers stood tall to the challenge to deliver their eighth victory in their last nine outings in all competitions.
Kevin Kelsy broke the deadlock inside the first-half from a Ryan Naderi assist, before the Czech international etched his name onto the scoresheet after the interval.
The duo were unplayable at the top end of the pitch, and Kelsy bagged his brace just minutes later when he slammed home from a corner.
And off the bench, Tochi Chukwuani put the seal on the win in the dying embers when he stroked home from a Djeidi Gassama pull-back.
From the team which played out a 2-2 draw with Dundee seven days ago, in came the Venezuelan international in place of Gassama.
Badredine Bouanani retained his place on the right-hand side following an impressive display, and the Algerian international shimmied with intent in the opening exchanges.
Inside four minutes the winger had already left a St. Johnstone jersey in his wake with a delightful shot, opening up the space for an effort which was deflected out for a corner.
Minutes later, a dangerous outswinging delivery sailed in the direction of Nico Raskin in a crowded penalty area, but the Belgian couldn’t quite make the required contact to head towards the St. Johnstone goal.
The home side ought to have broken the deadlock just shy of the quarter-hour mark when the lively Bouanani combined with Minsu. The South Korean thread the needle with a gorgeous return pass, but Bouanani was unable to dig out the finish with only the goalkeeper to beat.
Toby Steward in the visiting goal almost had egg on his face minutes later when Kelsy drove a Minsu lay-off goalwards and the St. Johnstone goalkeeper watched gratefully as he spilled his attempted save wide of the target.
Much like last weekend on Tayside, Bouanani appeared the most likely route to goal for the Ibrox side.
He was again the recipient of a Minsu pass on the right-hand side before manufacturing space for a thunderous effort which looked destined for the far corner before the intervention of Liam Smith.
And when the opener arrived for Rangers on 29 minutes, it was no more than they deserved.
Minsu was at the heart of the action following a regain from Raskin in midfield, lifting his head to find Naderi breaking beyond the St. Johnstone backline. The Czech international was unselfish in spotting Kelsy across the face of goal and after being teed up by his strike partner, the Venezuelan swept home his third goal in Rangers colours.
Derek McInnes’ side were purring, and they almost doubled their advantage when Minsu took it upon himself to dial up an effort from 25 yards out only for the woodwork to deny the attacker.
Quite how the scoreline remained only one was a mystery.
Kelsy pounced on a loose touch with Cheick Diabate in the St. Johnstone defensive ranks and suddenly a two-versus-one opened up with Naderi. The duo exchanged passes, but Kelsy’s attempted curling finish from 12 yards out was met by a terrific diving save from Steward.
As expected, the Perth outfit emerged for the second half with a renewed sense of energy.
Taylor Steven burst forward just minutes into the second period and was able to dig out a cut-back into the path of Smith whose first-time effort was thwarted by a combination of Raskin and Dujon Sterling.
Moments later, Diabate rose highest from a corner delivery but could only glance his header wide of the far post.
The hour-mark ticked along with the scoreline intact as Steven curled into the arms of Ivor Pandur from distance.
However, just as the rain began to reach biblical proportions inside Ibrox, so too did the second goal arrive for McInnes’ men.
Joel Cotterill of the visitors was caught in possession midway inside his own half, which freed the space for Raskin to feed Minsu advanced on the left. The winger, who was Rangers’ brightest spark, unleashed an effort on his left side, with his effort squirming under Steward and presenting Naderi with a tap-in.
The contest was then put beyond doubt with 25 minutes remaining.
A wicked corner delivery was flicked on by Emmanuel Fernandez, with Kelsy drifting into space at the far post where he caressed a lovely first-time volley beyond Steward.
St. Johnstone were forced to play the remaining 19 minutes with ten men when half-time substitute Kai Fotheringham was shown a straight red card following a VAR review for a nasty challenge on Raskin in midfield.
There was still time for a fourth from the Gers with just four minutes remaining when Gassama burst down the left-hand side in customary fashion and his pull-back was guided home by Chukwuani from point-blank range.
RANGERS: Pandur, Sterling (Godfrey, 79), Fernandez, Makhanya, Penrice, Neil (Rommens, 68), Raskin, Bouanani, Minsu (Miovski, 79), Kelsy (Chukwuani, 82), Naderi (Gassama, 68)
Subs not used: Kelly, Nedeljkovic, Dragojevic, Curtis
ST. JOHNSTONE: Steward, Alic, Diabate, Steven, Fowler (Schmidt, 45), Gullan (Chalmers, 63), Finlayson (Fotheringham, 45), Cotterill, Campbell (Mikulic, 45), Smith, McAlear (Holt, 79)
Subs not used: Sinclair, Foulds, Todd, Paton