The home team scored twice in the opening 15 minutes through Kenny McLean and Paul McGowan, and despite having Jim Goodwin dismissed the Buddies held their lead.
St Mirren got off to the best possible start as McLean crashed a half-volley past Ben Williams after Steven Thompson knocked down a cross from John McGinn with just 13 seconds on the clock.
And Hibs were in further trouble on 15 minutes when McLean turned provider sliding a weighted pass through to McGowan, who got in behind the napping defence and shot across Williams into the corner.
There was a moment of controversy after half-an-hour as Goodwin and Hibs midfielder Kevin Thomson challenged for a 50/50 ball, which resulted in a straight red for the St Mirren defender.
Thomson was on his feet seconds later, much to the ire of the home support, and he was involved in creating a wonderful chance for Sam Stanton on 41 minutes, but the youngster contrived to blast the ball over the bar from four yards.
Stanton had an earlier shot parried by St Mirren keeper Christopher Dillo and as the ball spun towards Danny Haynes, he was unable to take advantage.
After the break, St Mirren came close again when Michael Nelson's headed clearance dropped invitingly for Conor Newton and his powerful drive was held by Williams.
At the other end, Owain Tudur Jones climbed above the St Mirren defence to meet a corner, but the midfielder's header from 12 yards was held comfortably by Dillo.
Jason Cummings fired a dropping ball inches wide of the target in the dying minutes, but Hibs were unable to grab a consolation.