Harris nodded in at the far post from a Darren Wheeler cross with three minutes remaining to finally break the deadlock against a resolute Chalfont defence, which had worked hard to keep the Rebels at bay and looked set to take a point from the match.
Chalfont had the early pressure in the first half without creating a clear chance, and the first real test for either goalkeeper was for Michael Power-Simpson to parry away a strike from Jamie McClurg from 20 yards following good interplay.
Dan Burnell had a golden chance to put Slough ahead after one-time Rebels player Liam Fowler handled in the box, but he saw his weak penalty kick easily saved by Power-Simpson, who merely had to move to his left and fall onto the ball.
Ollie Burgess clipped the top of the bar after a Darren Wheeler cross was nodded down to him, and Ryan Lake had a goalbound header cleared off the line. The Chalfont goal was beginning to lead a charmed life as first Dave Woozley, then Burgess again had shots blocked, as Slough pressure intensified.
The half ended in disappointment for Slough as Sean Fraser was carried off with an injury, to be replaced by Wes Harrison. It was later confirmed that Fraser tweaked ankle ligaments as he stretched to try and intercept a pass, and will miss around four weeks. Chalfont almost compounded the disappointment before that by taking the lead, but Scott Fitzgerald headed a good chance wide.
As the second half progressed, Slough created the clearer chances, with the hosts threatening rarely – but Chalfont defended robustly and in numbers. Warren Harris headed a half chance wide before Scott Harris entered the fray, replacing McClurg as Slough stepped up their search for a goal.
Slough continued to be frustrated, with a series of Darren Wheeler deliveries being repelled, and George Short headed wide from a Guy Hollis long throw.
Nathan Jack replaced a tiring Dan Burnell to make his debut, and after Burgess saw his header comfortably saved by Power-Simpson, it looked as it Chalfont would earn the point they were holding out for – but with 87 minutes on the clock, Power-Simpson misjudged the flight of Wheeler’s cross and Warren Harris was on hand to seal all three points for Slough, as reward for their perseverance.
Chalfont St Peter Line up
Michael Power-Simpson, Kevin Ayodele, Ronayne Marsh-Brown, Gareth Paxton, Liam Fowler, Alan Hedley (c), Romaine Walker, Davis Haule, Terrell Lewis, Scott Fitzgerald, Asen Valchev. Subs: Alex Paine, Chris Christie, Luke Brook-Smith, Danilson Vicente, Remmel Clarke.