Stuart Kettlewell also got in on the act with a cracking strike as all the goals came inside the first hour in Dingwall, leaving winless St Mirren second from bottom.
Gary Harkins latched onto a Marc McAusland knockdown early on but County keeper Mark Brown was alert to the danger and the action soon swung back down the other end.
Out of nothing Kettlewell (8) took aim from 20 yards and blasted a tremendous strike into the roof of the net to bring the home fans to their feet.
Conor Newton came close with a decent effort as St Mirren pressed while Kevin Luckassen showed some neat skill but could not keep his shot on target.
County doubled their advantage (23) after Luckassen was brought down by McAusland in the box. Brittain's penalty was parried by David Cornell but the County captain netted at the second attempt.
The home side - who had conceded nine goals in their first three league games - were oozing confidence with Graham Carey and Luckassen both carrying a potent threat.
The third goal finally arrived in the 56th minute as Carey's short corner teed up Brittain, who worked some space before drilling the ball into the far corner and there was no way back for Saints.