The tie had looked destined for a replay on Tuesday evening, with Slough unable to penetrate the home side's defence despite having the better of the play – but in the final minute, Wanderers captain Chris Boulter slid in to block a Warren Harris shot and used his hands, resulting in a penalty being awarded.
Adam Martin stepped up and sent Slavomir Huk the wrong way from the spot to send the Rebels through to the next round.
Slough arrived at the extremely scenic ground at Westhumble hoping to put right the wrongs of Tuesday night's county cup exit against Burnham, and also reverse recent fortune in the world's oldest domestic cup competition, having suffered early exits in the last two seasons. However, standing before the Rebels was a Dorking Wanderers side in good form so far this season, unbeaten and sitting second in the Isthmian League Division One South.
Both goalkeepers were tested in the opening three minutes, as first Huk kept out a Gary MacDonald header, before Luke Williams pushed a long-range strike wide of the post.
Mpi saw a headed effort ruled out for offside, and just a minute later Huk saved well with his legs from the tall striker, after he was set up by partner Eddie Smith.
Referee Davis awarded Dorking an indirect free kick in the Slough penalty area for an alleged back pass offence five minutes later, which saw Guy Hollis and Jamie McClurg both booked for dissent, but Slough held firm and blocked both the initial shot and a follow-up effort.
Eddie Smith was at the heart of Slough's attacking threat, but was a frustrated figure and again departed the action early after suffering another pulled hamstring in the 36th minute. Earlier, he had been denied a penalty after going to ground under a challenge, and saw a shot saved easily by Huk after being played in by Nathan Webb.
Before this, Slough were twice thwarted by fantastic saves from Huk to keep the game goalless. MacDonald's headed effort from a deep corner was clawed away by the keeper, who then recovered to keep out Webb's headed follow-up on the goal line. Charlie Mpi had a shot deflected over in the ensuing scramble.
For Dorking, Phil Page's long-range shot, which flew a few yards wide, was the closest they came to troubling Williams again in the first half.
Huk was the hero again early in the second half, keeping out substitute Lewis Putman's side-footed shot after Webb had done well to bring the ball into the box and pull it across.
Wanderers threatened only sporadically and mainly on the counter-attack. Kev Terry charged into the Slough box on 53 minutes, but his touch was too heavy and allowed Williams to rush out and block.
After a strong start to the second half, Slough's attacking threat was in danger of fizzling out as the half wore on. Adam Martin struck a low shot straight at Huk, and whilst Slough carried the greater threat and worked hard to pressure the home side into making mistakes, the Rebels could not find a path to goal.
Their best source of ammunition looked to be through Putman, who often found himself linking up with Webb on the left, and between them they often got the better of their opponents, but could not create a clear chance from crosses into the box.
Scott Harris replaced Mpi as Slough freshened things up, but the visitors continued to be frustrated until the clock struck 90 minutes when Boulter was penalised for handball, leaving Martin to fire Slough into the lead.
Williams needed to be alert one last time as he saved a Boulter header down by his post, but Slough saw out the remaining seconds to record a victory which was deserved overall.
Slough Town Line Up
- 1 Luke Williams
- 2 Sean Fraser
- 3 Ryan Parsons
- 4 Guy Hollis
- 5 Gary MacDonald
- 6 Nathan Webb
- 7 Jamie McClurg
- 8 Adam Martin
- 9 Charlie Mpi 18
- 10 Edward Smith 16
- 11 Warren Harris
Dorking Wanderers Line up
Slavomir Huk, Luke Hackett, Jake Hill, Chris Boulter (c), Ben Dyett, Tony Oaks, Jerome Beckles, Daryl Coleman, Kev Terry, Phil Page, James Duff. Subs: Elliott Thompson, Shaun Findlay, Harry Shipton, Alex Rose, Jordan Willis.