Saints looked on course for their first home win of the season thanks to Jack Baird’s early header but it was snatched from them in the last minute when Alex Samuel tapped in.
Morton went into the game on the back of four consecutive wins and they made the early running with a couple of corners and a fizzing drive from Stefan McCluskey.
However, it was the home side who took the lead on 12 minutes and the goal was a disaster for visiting goalkeeper Grant Adam. The shot-stopper let a Scott Agnew corner slip through his hands and Baird headed in from a couple of yards out.
As Morton looked to hit back quickly McCluskey was booked for simulation as he went down after a challenge from Jamie Langfield who had darted from his goal.
McCluskey almost took his anger at the decision out in the best possible way a minute later with a curling shot that just cleared Langfield’s bar.
Morton came within an inch of a leveller on 34 minutes when Ross Forbes’ shot was parried by Langfield but looked set to go in or be touched home by Denny Johnstone until Craig Reid made an overhead clearance.
Adam went some way to atoning for his earlier error with a fine save from a net-bound Stephen Mallan free-kick six minutes from the break.
Th first effort of note in the second half came on 67 minutes when Johnstone sent in a shot that beat Langfield, however it nestled in the side-netting. Lawrence Shankland headed a Mallan free-kick inches wide on 72 minutes with the visiting Thomas O’Ware doing likewise with a Bobby Barr cross six minutes later.
Morton thought that it was not going to be their night until Johnstone fired in low from 18 yards and when Langfield could only push the ball out substitute Samuel was on hand to net.
St Mirren: Langfield, Kelly, Webster, McMullan (McLear 74), Carswell, Mallan, Baird, Agnew (Howieson 89), Gallagher, Shankland (Thompson 86), Reid. Subs not used: Wilks, Conlan, Morgan, Cuddihy.
Morton: Adam, Kilday, O’Ware, Forbes, Johnstone, McCluskey (Samuel 75), Tidser (McKee 83), Pepper, Barr, Russell, Gasporotto. Subs not used: McGowan, Miller, Scullion, Orr, Stevenson.