Well-organised St Mirren took advantage of an off-day for Hibs to snatch a deserved three points.
Mehmet celebrates opening the scoring at Easter Road
A dreadful mix-up between captain Rob Jones and goalkeeper Yves Ma-Kalambay gifted Billy Mehmet a strike into an empty net on 12 minutes.
Hibs were reduced to 10 men when second half substitute Dean Shiels was sent off after a tangle with Franco Miranda.
And, with the home side posing little attacking threat, the visitors from Paisley held on comfortably.
A second victory in a row lifts St Mirren safely into mid-table, while a second consecutive defeat has brought Hibs' bright start to the season to a disappointing halt.
Hibs were lacklustre throughout but were denied an early claim for a penalty when Saints defender Andy Millen had not one but two tugs at the shirt of Mickael Antoine-Curier.
The home side then conceded a ridiculous goal when Jones and Ma-Kalambay made a mess of dealing with a hopeful ball forward and it fell to Mehmet, who found the empty net via a post from 20 yards.
With the goalkeeper advancing to collect, skipper Jones, under no pressure, nodded the ball back and it rebounded off the startled Ma-Kalambay to present the St Mirren forward with the easiest of chances.
With St Mirren working hard to close them down, Hibs struggled to find any passing fluency and the crowd were growing restless at the scrappy nature of the game.
Visiting goalkeeper Chris Smith was lucky to escape when he came to gather a corner kick and dropped it at the feet of Clayton Donaldson but the ball popped back into his grateful arms, with the striker slow to react.
Ma-Kalambay was unconvincing again when he fumbled a headed back-pass from David Murphy to give away a corner, from which Stewart Kean's deflected shot gave the home side a scare.
Hibs introduced Shiels and Alan O'Brien after the interval, with Merouane Zemmama and the sluggish Antoine-Curier making way.
Smith again found himself in trouble when he missed another delivery into his area but Brian Kerr could only blaze high over the target on the half-volley from eight yards.
After 57 minutes, Smith made his first save when he had to look smart to keep out a shot from Lewis Stevenson.
The Hibs teenager then slashed an effort horribly wide after doing well to work the ball on to his left foot inside the penalty box.
Things went from bad to worse for John Collins' out-of-sorts team when Shiels was shown a red card for aiming a kick at Miranda.
With a man advantage, St Mirren coped easily with most of Hibs' attacks, and when the ball fell to Kerr in a promising position the midfielder again fired over.
It has taken the Buddies 17 years to win at Easter Road but the visitors can seldom have had a more comfortable afternoon.
Hibernian: Ma-Kalambay, McCann (Campbell 64), Hogg, Jones, Murphy, Zemmama (O'Brien 46), Kerr, Chisholm, Stevenson, Donaldson, Antoine-Curier (Shiels 46). Subs Not Used: McNeil, Gathuessi, McCormack, Lynch.
Sent Off: Shiels (65).
Booked: Jones, Chisholm.
St Mirren: Smith, van Zanten, Haining, Millen, Miranda, Brady, Mason, Murray, Burke (McGinn 61), Mehmet, Kean. Subs Not Used: Howard, Potter, Corcoran, McCay, Barron, Birchall.
Booked: Brady, Mason, Miranda.
Goal: Mehmet 13.
Ref: C Boyle