A Liam Boyce hat-trick ensured that Ross County would extend their unbeaten run to nine games and all but confirm relegation for St Mirren.
Another disappointing home display by St Mirren was capped off when Bulgarian defender Viktor Genev was shown a straight red card just after the break.
Boyce took advantage of some poor St Mirren defending to bag his first goal and struck twice in the second half to become the first Ross County player to score a hat-trick in the Scottish top flight.
County took a deserved lead after 29 minutes.
On-loan Michael Gardyne had his shot charged down by Jim Goodwin and the ball fell to Craig Curran who saw his close range effort saved by Mark Ridgers, but the ball fell perfectly for the in- form Boyce and he crashed in his second goal in as many games.
Ridgers kept the Buddies in the game with some fine saves, while Gardyne flashed a volley a yard over the bar.
The Staggies scored a second on 74 minutes. Gardyne whipped in an inviting cross and Boyce showed great composure to beat Goodwin before unleashing an unstoppable strike for his second goal.
Boyce claimed the match ball after he tapped home a Tony Dingwall cross at the far post.