Liam Boyce netted a hat-trick for in-form County, who won more comfortably than the score suggests against a St Mirren side who played for just over 40 minutes with one man less following a red card for Viktor Genev.
The visitors should have gone ahead on 26 minutes when Michael Gardyne beat the offside trap by a yard and rounded the advancing Marc Ridgers to slot home, but he was denied by the assistant’s flag.
A minute later Craig Curran sent a low delivery across the penalty area and St Mirren defender Jason Naismith turned the ball against his own post and wide for a corner.
But the goal arrived when Gardyne weaved into the box and fired a shot towards goal which was blocked by Jim Goodwin – Ridgers reacted instinctively to parry the rebound from Curran but the ball dropped to Boyce (30) and he smashed it over the line from six yards.
Boyce fired another shot a couple of yards wide of the target five minutes before the interval when he was allowed to turn at the edge of the St Mirren penalty area unopposed.
And Martin Woods was next to have a go from a similar distance, playing a one-two with Gardyne and sending a dipping shot over the bar.
St Mirren’s hopes of turning the match suffered a major blow four minutes after the interval as last-man Genev bundled Boyce to the deck as the County striker raced through on goal and the defender was dismissed.
Gardyne curled the resulting free-kick from 30 yards round the St Mirren defensive wall and off the ‘keeper’s left post.
From slightly further out Woods curled another free-kick towards the opposite corner which forced Ridgers to turn round the post for a corner.
Two minutes later Ridgers produced a better save to keep out a powerful shot from Raffaele De Vita after Boyce laid the ball back to the edge of the box.
On the hour County goalkeeper Mark Brown had to be alert when John McGinn exchanged passes with Stephen Mallan and crashed a dipping drive towards goal from 25 yards.
Back at the other end of the pitch Gardyne curled a shot narrowly wide of the upright from 15 yards and Boyce later sent a glancing header over the bar from a Woods corner.
But Boyce secured the points with 15 minutes remaining, taking a cross from Gardyne on his chest and slipping the ball through Goodwin’s legs before blasting his shot high into the net from eight yards.
Boyce completed his treble in stoppage-time when Jackson Irvine threaded a pass to substitute Tony Dingwall and he squared across the six-yard box for the Northern Irishman to clip over the line.
The victory means County can no longer be automatically relegated from the Scottish Premiership and after opening up a six-point gap with second bottom Motherwell, they are also firm favourite to avoid the playoff lottery.