Danny Lennon's men had dominated the first half but looked set for a draw as they toiled to breakdown a determined Staggies defence.
But substitute Wylde rammed home a 20-yard beauty into the top corner to hand Saints a third win in four games.
They now have a four-point advantage over second-bottom Kilmarnock - who play Hearts on Sunday - with two games remaining.
County, though, must hope the Ayrshiremen do not win at Tynecastle or they will drop into the relegation play-off spot ahead of the season's climax.
Saints started like they meant business and forced Mark Brown into a fourth-minute save when Kenny McLean swept a decent free-kick round the County wall.
The Buddies' midfielder then had Brown scrambling on 18 minutes as his well-hit volley slipped from the keeper's grasp. John McGinn raced in looking to pounce but Brown did well to snuff out the danger as he dived on the loose ball.
Paul McGowan then got a clever flick to Sean Kelly's front-post cross moments later but Brown was again well paced for the save.
Saints continued to look the more dangerous team as the match ticked past the half-four, with McLean usually central to the Paisley side's brightest moments.
But the Buddies were grateful for the nick Melvin de Leeuw's shot took off Jim Goodwin's shin as it squirmed wide of Chris Dilo's right-hand post seven minutes before the break.
The second half began with another close effort for St Mirren when Jason Naismith's inviting low ball was allowed to run all the way across the six-yard box without finding a taker.
Despite the vast majority of possession though, unlocking the Staggies' defence was proving difficult for Lennon's team. And they nearly paid for their lack of cutting edge when former Saint Richie Brittain curled a free-kick onto the face of Dilo's crossbar on 57 minutes.
And the Highlanders should have scored moments later when de Leeuw cut the ball back for Alex Cooper - a half-time replacement for Graham Carey - only for the forward to strike the ball with his ankle when he had hoped to connect with his laces.
The balance of the game had now evened itself out and Cooper had a second go after cutting inside Darren McGregor following a swift County counter-attack, but sent his shot straight at Dilo.
Saints drove up the park almost immediately after that chance but saw Brown respond well to Conor Newton's low drive from 18 yards.
Cooper had a third chance after a smart one-two with Brittain cut through the Saints defence on 67 minutes but was again spurned by Dilo's reactions.
St Mirren had pushed on Wylde and Josh Magennis for McGowan and McGinn as they looked to freshen things up.
But frontman Magennis hardly did Stephen Thompson any favours with a drilled cross that was a yard behind his strike partner and the best the former Scotland international could do was scoop his header over the County bar.
As the game moved into the final quarter, a draw was looking increasingly likely as County's stuffy tactics frustrated the hosts.
But with 12 minutes remaining Wylde was given space to shoot 20 yards out and punished the visitors' casualness with a stunning top-corner winner.
Kenny McLean could have made sure of the win in the last 10 minutes but first sent a low shot wide after driving into the box from the left before smashing a post with a dipping volley from far out on the other side.
There was still time for Ross County to give Saints a scare though as Rocco Quinn volleyed onto the bar with a minute of normal time remaining.