With both teams requiring the points to ease their respective relegation fears, this was one neither could afford to lose.
The Saints have not won in the SPL this year and they almost fell behind in the first few minutes when Dougie Imrie rifled an ambitious dipping volley a yard over the crossbar.
It was the visitors, however, who took the lead after six minutes with a goal that owed much to Caley Thistle's poor defending as good attacking play from St Mirren.
Billy Mehmet's flick-on was collected by Craig Dargo and although he lost possession, the ball rebounded off Lionel Djebi-Zadi and kindly into the path of an unmarked Andy Dorman, who took full advantage of both the hesitancy of Grant Munro and Pavels Mihadjuks to stroke the ball into the corner past Michael Fraser.
A spell of misplaced passes by Inverness continued to frustrate the home support with only an attempt by Grant Munro on 25 minutes from Imrie's corner that was blocked by goalkeeper Mark Howard bringing brief applause.
Seven minutes later, Imrie was unfortunate not to register his first goal in ten games as his effort fizzed a couple of inches past the upright, much to the relief of Howard.
The Saints number one, however, was not so fortunate five minutes before the interval as Caley Thistle restored parity.
Munro's raking crossfield pass found David Proctor who flicked it first time on to Imrie. He wasted no time in setting up Morais, who quickly adjusted his body before sending a first-time volley past Howard and into the bottom corner.
That goal clearly gave Morais a taste for it as two minutes after half-time he was at it again, this time turning superbly 15 yards out and sending an unstoppable effort beyond Howard at his near post.
St Mirren's response was to introduce Stephen McGinn and then Jim Hamilton into the action and it was the former who went close with a near-post header.
Hamilton was then next to have a go on 75 minutes with a drive that Fraser spilled in front of Dorman, but from only a few yards out he crashed his shot off the bar.
The visitors pushed hard for an equaliser, yet it was Caley Thistle who emerged with the vital three points.