Hibernian's relegation problems were compounded as St Mirren held on for victory thanks to a Kenny McLean strike as both sides finished with 10 men.
Eoin Doyle missed two great chances for Hibs either side of the break.
McLean slotted a low drive into the corner of the net after 65 minutes.
Buddies defender Marc McAusland was sent off 13 minutes later for a second yellow card, but Hibs were unable to make their advantage tell and Lewis Stevenson saw red in stoppage time.
With that departure went Hibs' last chance of an equaliser, leaving the Edinburgh side only three points ahead of Dunfermline Athletic, who won their first home game of the season by beating Aberdeen 24 hours earlier.
Hibs had been denied the services of top scorer Garry O'Connor through illness, so Doyle joined George Francomb and the fit-again Tom Soares in starting, with Ivan Sproule and Jorge Claros dropped to the bench.
St Mirren, for whom left-back Jeroen Tesselaar returned from suspension to replace Dougie Imrie, threatened first with a 25-yard Jim Goodwin drive being deflected wide.
But Hibs settled and Leigh Griffiths forced Craig Samson to concede a corner with a low shot towards the goalkeeper's near post from a tight angle.
Francomb squandered a decent opportunity when he fired well over from the edge of the penalty box after the ball fell to him in space following a corner.
Last-week's hat-trick hero for the Buddies, Steven Thompson, headed Graham Carey's deep inswinging corner just wide from eight yards.
Griffiths sent a powerful, 25-yard drive curling wide of the far post, while Hibs defender James McPake had to head clear from in front of his goal after a fine run and cross from Nigel Hasselbaink.
Hibs missed a great chance to end the half on top when Doyle ran unchallenged onto a Matt Doherty through ball, but the Irishman lobbed the ball well wide with Samson having exposed himself by rushing well wide of his unguarded goal.
St Mirren winger Carey finally forced Mark Brown into a save with a drive shortly after the break, but it was Hibs who were now the more dangerous.
Doyle pounced on a loose ball in the Buddies defence, evaded a couple of challenges but from 12 yards fired straight at Samson, who managed to push the ball wide for a corner.
Hibs paid for those misses when Tesselaar broke down the left before playing it into the path of McLean to sidefoot home from the edge of the box.
Hibs claimed for a penalty when Griffiths was tackled by Imrie, but the substitute appeared to make contact with the ball.
St Mirren were looking comfortable but were reduced to 10 men when McAusland was shown a second yellow card for pulling back Griffiths.
With the home side content to hold on, Stevenson also left the scene after a second booking, ruling out the versatile midfielder for at least one vital game of the run-in.