St Mirren eased their relegation fears and dealt a blow to caretaker Tommy Craig's hopes of becoming Hibernian boss with a comfortable win in Paisley.
Mason made sure of the points for the Buddies
But it was a freak goal from Ian Maxwell that gave the Buddies the lead after five minutes, the defender's cross looping into the net.
Garry Brady and John Potter threatened Hibs again before the second goal, Gary Mason firing home after 43 minutes.
Mickael Antoine-Curier backheeled a late reply for lacklustre Hibs.
But the Buddies held on to move above Hearts and into 10th spot in the Scottish Premier League.
Andy Dorman, who had received international clearance after his move from New England Revolution this week, had to make do with a place on the St Mirren bench.
But Brady, Will Haining and David Barron did make the starting line-up in place of Alan Reid, Franco Miranda and Stephen McGinn.
Hibs also had three changes, with Guillaume Beuzelin, Alan O'Brien and Mickael Antoine-Curier replacing Dean Shiels, Patrick Noubissie and Filipe Morais.
Early lethargy was broken in the fifth minute, when the home side took the lead with Maxwell's first goal of the season.
The Buddies left-back aimed a curling cross deep into the Hibs penalty area, but the ball soared over Easter Road goalkeeper Yves Ma-Kalambay and went in off the far post.
Haining was inches away from getting on the end of Brady's free-kick as St Mirren went close to a second goal.
Hibs' first sign of a response came in the 21st minute, when Antoine-Curier volleyed wide from the edge of the box.
Maxwell gave St Mirren a fortunate lead
But the home side retained the upper hand, with a Brady snap-shot from 30 yards flying a yard over and Ma-Kalambay saving well from a John Potter header.
St Mirren doubled their lead two minutes before the break after a mistake by Rob Jones.
The defender was short with a back-pss, Ma-Kalambay blocked Stewart Kean's drive, but the ball broke to the striker and his lay-off was hammered home by Mason.
Hibs, who were booed off by their fans at the interval, made one change at the break, striker Clayton Donaldson being replaced with Damon Gray.
The visitors' work rate increased, but there was little to suggest they could rescue the game.
Beuzelin and Ross Chisholm both had weak efforts in quick succession as St Mirren concentrated on defending their lead.
St Mirren were still dangerous on the break and Mason fired just wide from long range.
Antoine-Curier grabbed a goal back for Hibs when he took a Chisholm long ball, rounded goalkeeper Chris Smith and knocked the ball in with his heel after 88 minutes.
But it was not enough to prevent Hibs going eight games without a win.
St Mirren: Smith, van Zanten, Haining, Potter, Maxwell, Barron, Mason, Brady (McGinn 66), Burke (Miranda 73), Mehmet (Dargo 89), Kean.
Subs Not Used: Howard, Dorman, Corcoran, McCay.
Goals: Maxwell 4, Mason 43.
Hibernian: Ma-Kalambay, Gathuessi (McCormack 67), Jones, Hogg, Stevenson, Chisholm, Beuzelin, Kerr (Morais 71), O'Brien, Donaldson (Gray 46), Antoine-Curier.
Subs Not Used: McNeil, Noubissie, Lynch, Shiels.
Goal: Antoine-Curier 89.
Ref: K Clark