The Easter Road men were in the race for a top-six slot just last month but now find themselves in real danger of the drop after defeat in Paisley.
Kenny McLean fired the Buddies ahead with an instant opener before setting up Paul McGowan for the second after 14 minutes.
Hibs were given an opening when Saints skipper Jim Goodwin was shown a straight red card for a crude lunge on Kevin Thomson, but even with a man advantage they still could not find the net.
Terry Butcher's side have now failed to score in 17 of their 34 matches - more than any other side in the Scottish Premiership - and the boss was an angry figure on the touchline as he reacted with fury every time his team made a mess of it.
They are now joined in joint-seventh place by the Buddies but have just three points cushioning them from Ross County in the relegation play-off zone.
Butcher's problem is that he does not trust many of the players he inherited from Pat Fenlon and finding a settled team is a feat that continues to allude him.
The former England captain has already admitted he has no idea what his best line-up is and made another four changes for the Paisley trip as Liam Craig, Tom Taiwo, Paul Cairney and the suspended Ryan McGivern dropped out following the 2-0 defeat to Aberdeen two weeks ago.
With striker Paul Heffernan out for the season with a groin injury, Butcher instead gave Owain Tudur Jones, Alan Maybury, Danny Haynes and Thomson their turn to impress - it did not work.
By contrast, Danny Lennon named the same St Mirren team to the one who shocked Motherwell with a 3-2 win last time out.
And the confidence that result produced was still coursing through the Buddies' veins as they opened the scoring before Hibs had even touched the ball.
John McGinn received the ball straight from the kick-off and lumped a long pass up to Steven Thompson. Towering over Michael Nelson, the former Scotland striker planted a perfect knock-down at the feet of the on-rushing McLean, who beat Ben Williams with a low drive.
Saints continued to strut and grabbed a second just before the quarter-hour mark. Hibs youngster Jordon Foster was caught coming out of defence by Goodwin, who fed it to McLean and the scorer of the first slid in McGowan to fire home the second.
Hibs looked stunned but after giving themselves a shake did at least test Chris Dilo with Sam Stanton's 25-yard strike, which the keeper had to gather at the second attempt as Haynes rushed in.
Goodwin, though, then handed the initiative back to the Edinburgh men as he slid in over the ball to catch Thomson late on the ankle. The home supporters were furious as the red card was flashed but referee Kevin Clancy was absolutely spot on.
Yet Hibs still struggled to make progress. Tudur Jones sent a free-kick into the Buddies wall, while Stanton blazed over from eight yards out after McGowan was robbed of possession.
It could have got worse when Sean Kelly flashed a low shot just past the far post with the last kick of the half.
The visitors were kept waiting as Saints took their time returning after the break and found themselves frustrated further as some stout defending blocked their attempts to find a way back into the match.
At the other end, Conor Newton's 55th-minute volley was well struck but straight at Williams.
Hibs' extra man was coming in useful as they dominated possession but there was still no spark as they toiled to break the 10 men down.
Alex Harris saw a half-volley easily saved by Dilo with 15 minutes left, but Butcher's men were left breathing a huge sigh of relief as a goal by Saints sub Josh Magennis was ruled out after Marc McAusland gave Nelson a push in the build-up.
Hibs replacement Duncan Watmore missed with two chances inside a minute but Saints stood solid to claim just their fourth win in 18 games.