The three points ensured the hosts, who remain unbeaten this year, climbed to eighth and within just two points of fourth spot.
Northern Irishman Sproule was the man who made the difference with a lively showing at the Global Energy Stadium and it was all the more surprising with the winger a major doubt for the encounter with a hamstring problem.
The visitors had also been in impressive form and having started the match in fourth place they too would have had designs on the top positions but winger Sproule and his early strike on 11 minutes ensured it was the hosts celebrating.
Saints started on the front foot and Mehdi Abeid tested the brilliant Mark Brown with a low drive, but it was County who took the lead (11) with their first meaningful attack as Sproule picked up a pass from skipper Richard Brittain to curl in a delicious shot to give his side the edge.
The visitors stormed back and Steven Maclean went close to restoring parity (16) when he beat his marker and fired a shot towards the far corner and it needed a spectacular one-handed save from Brown to keep it out.
Brown was becoming ever-busier and parried Michael Kovacevic's effort and it would have been the hosts heading in at the interval relieved to have their advantage undamaged.
After the break the Saints continued to pour forward and it took a timely intervention from Grant Munro to stop Rowan Vine after the striker played a one-two with Abeid.
Whenever Brown was called upon, he did not disappoint and shut out Liam Craig with his legs on 55 minutes.
Finally County were reignited and Sproule twice went close, before Brittain wasted a chance to score against his former employers when his free-kick (77) flew narrowly wide.
The hosts were hanging on desperately as Craig fired wide in injury-time, but they held on to produce a very satisfying three points and breathe fire into their quest for a place in the top four.