De Leeuw's winner arrived seven minutes from time after St Mirren midfielder John McGinn had cancelled out a Yoann Arquin goal.
Striker Arquin used his pace to get past Jim Goodwin and deliver a low centre into the box for De Leeuw to clip the ball into the roof of the net.
County opened the scoring on 38 minutes when Graham Carey and Michael Tidser combined to set-up Arquin and he beat the keeper with a looping header from 12 yards.
St Mirren levelled the match on 66 minutes when McGinn shuffled the ball from his left foot to his right and curled the ball high into the net from the edge of the box as County struggled to clear a Gregg Wylde corner.
The visitors created the first chance after 12 minutes when Wylde cut in from the left flank and tested the goalkeeper from inside the penalty area.
But Brian McLean should have given the hosts the lead when he met a free-kick from Richard Brittain and headed straight at Marian Kello.
County keeper Mark Brown was again called into action to tip Kenny McLean's dipping shot from 40 yards over the bar.
At the other end Kello looked on in relief as a shot from Jordan Slew spun over the St Mirren keeper and landed on the top of the net.
Arquin should have doubled his tally before the break when Yann Songo'o found him unmarked in front of goal, but he lost control with a poor first touch and his shot was blocked.