Rangers picked up the victory which ensured a maximum six points from the Christmas programme and meant they had lost no further ground on leaders Celtic.
Neil McCann, Fernando Ricksen and substitute Tore Andre Flo were the men on target on the day when West Ham boss Harry Redknapp publicly confirmed he was interested in deposed skipper Lorenzo Amoruso.
This was a match that took a while to lift itself out of the fog that enveloped Ibrox and it took an opposition move to do that.
Saints defender Darren Dods, who had been kept busy at the back, came forward to plant a header from Momo Sylla's corner on to the top of the Rangers' crossbar.
That served as a wake up call to the defending champions. Ronald de Boer was denied a second goal in as many games when his overhead kick was deflected wide by Gary Bollan.
Claudio Reyna was the architect of Rangers first goal, floating in a cross which appeared to be flicked on by Jim Weir ahead of de Boer, with Weir recovering to go on to make a fruitless lunge to stop McCann controlling and lashing the loose ball past Main.
It was another defender who made sure Rangers collected maximum points from the festive period with his first goal for the club.
Ricksen it was who popped up unmarked at the far post after McCann's cross had been flicked on by Miller.
That was that as far as a meaningful contest was concerned although Stefan Klos was called upon to divert a Sylla free kick over his crossbar.
Johnston almost made it three with a floated effort that drifted just wide but then he did make a difference with a run and cross over Main that was nodded in by Flo, who had come on for de Boer in the 66th minute.
Rangers: Klos, Ricksen, Konterman, Amoruso, Numan, Reyna, Ferguson, Tugay, McCann (Johnston 72), de Boer (Flo 66), Mols (Miller 28). Subs Not Used: Christensen, Ross. Goals: McCann 32, Ricksen 54, Flo 87.
St Johnstone: Main, Sylla, Dods, McCluskey (McCulloch 58), Weir, Bollan, McBride, Kane (Dasovic 46), O'Neill., Lovenkrands, Parker. Subs Not Used: Cuthbert, Connolly, Forsyth. Booked: Weir.
Referee: M McCurry (Scotland).
