Richard Brittain fired the visitors ahead with an excellent 25-yard free-kick and the Highland side passed up numerous opportunities to extend their advantage during the first half.
St Mirren were rewarded for a bright start to the second half when Steven Thompson rocketed a shot past Mark Brown from just inside the area, but it was to be the highlight of the afternoon for Danny Lennon's side.
Sam Morrow took advantage on a horrific defensive error by David Barron to restore County's lead and Ivan Sproule marked his debut with a goal four minutes later.
Sproule capped off an excellent display when he tapped home a fourth and a miserable day for St Mirren was completed when Lewis Guy's penalty was saved by Brown in injury time.
Craig Samson had earlier made a fine save for the hosts, diving full stretch to keep out Iain Vigurs' free-kick, while Gary Teale forced Brown into a good stop just before their late penalty as St Mirren looked for a consolation.