Things had started so brightly for Slough, as in the seventh minute, Adam Martin ran on to Dave Woozley's knock-down to fire past James Reading to put the visitors ahead.
Slough could have doubled the lead soon after, but Ed Smith fired wide after a flowing move, after being fed by Ollie Burgess.
But Slough could not get a stranglehold on the game, and Northwood equalised on 27 minutes. Jamie Jackson, making his 100th appearance for the club, had a moment to forget as Niko Muir was credited with bundling the ball in after a corner, though it looked more of an own goal to many.
Ryan O'Toole put a free kick just wide after Dave Deeney was fouled on the edge of the box, but the two sides went in level at half time.
Chances went begging for both sides in a second half which saw Slough become increasingly frustrated and anxious to score a winning goal, and they were to pay the price at the end.
Jon-Jo Bates shot over from a good position as Northwood broke away, and Slough struggled to create openings initially, but were later guilty of squandering some gilt-edged chances.
First, Slough worked the ball out to Burgess just inside the box, but his shot was saved by the legs of Reading. In the follow-up, Jake Parsons chipped the ball in to the middle, and Smith had an effort cleared off the line.
Just a minute later, Smith missed another glorious chance. After Reading kept out a headed effort, Smith could not force in the rebound, with Reading recovering well to turn the ball away.
Meanwhile, Northwood always carried a threat on the break, but their finishing was largely wayward. Smith's frustrating night came to end when he and Burgess were replaced by Dan Burnell and Adam Logie, and soon after, Burnell was denied what looked a certain goal as full-back Ryan Phillips cleared the ball away after Adam Martin, by far Slough's best player on the night, had beaten his man to create the chance.
Kyle Anthony headed a chance straight at Reading and at the other end, Northwood could have snatched the lead but Oliver Hawkins shot straight at Jackson.
But deep into stoppage time, Slough were caught out by a counter-attack, and Lee Grant found himself with space to fire in a shot from outside the box, which deceived Jackson and flew straight past him.
There was still time for the visitors to force a corner from which Anthony fired in a shot which cleared the bar only by a foot, but time ran out on a distraught Slough, with Grant's goal a big blow to their promotion hopes. Following their second league defeat in the space of just three days, the Rebels travel to fifth-placed Royston on Saturday in what now looks to be a must-win game.
Northwood Line up
James Reading, Lee Grant, Ryan Phillips, Brad Hewitt, Reece Yorke, Mark Burgess, Jon-Jo Bates (Craig White), James Burgess, Niko Muir, Juan Onieva (Oliver Hawkins), Hallelujah Basmel (Reon Thomas). Subs not used: Adil Salih, Travonn St Leonce.