Josias Carbon was next up to test the visiting keeper with a stinging drive from outside the area and Ross flung himself to his right to turn the ball behind.
Despite a shaky start, the visitors got themselves back into the game and Clemente Lopez had a good opportunity to put Harrow in front. The ball fell to the midfielder after a scramble in the Slough area and his sidefooted effort was well blocked by Avery.
Slough took the lead through Carbon in the twenty third minute. A long throw from Andy Ballard from the left hand side was flicked on by Ian Hodges and Carbon was able to bundle the ball home from close range.
Slough were lucky the scores were not levelled when a poor Masters clearance was weakly fed back into goal, with Mark Avery covering, and the keeper was able to collect easily.
Just before half time, McCleary got Slough's second, capitalising on a poor clearance from Ross. In fairness to the Harrow keeper, the ball bobbled horribly just as it approached him but his attempted clearance fell straight to McCleary by the right hand edge of the area and the Slough forward simply lofted it back over the keeper and into the net.
Harrow pulled one back in the fifth minute of the second half. A counter saw Brian Haule fed in down the right wing. His first time cross was met perfectly by the head of Elliot Onochie as he headed across goal and into the corner.
Mason restored Slough's two goal advantage a few minutes later. Darron Wilkinson fed a high ball forwards and it bounced up and Leigh Mason hooked it goalwards. Ross, who had looked shaky with aerial balls all game, misjudged the flight partially and the ball sailed over his head and in.
Harrow had a few chances with Steve Dogbe and Onochie pulling routine saves from Masters but Slough were next to score. Hodges had been played through and was taken down inside the area by Daniel Blanchett. The striker converted low into the bottom right corner to put Slough three in front.
Harrow pulled one back with the next chance of the game as Onochie notched his second of the game. A low cross from the right was perfectly into the path of sub Daniel McGonigle. His sidefooted effort was saved low to Masters right but the keeper could not keep the ball in his grasp and Onochie was able to prod the ball home from inside the six yard box.
In the final minute, Slough got their fifth. A cross was cleared and headed back in towards goal by Michael Alexis. Hodges ran on and chested the ball towards goal and Ross let the ball slip between his hands to round off a miserable evening for the keeper.
Harrow Borough Line up
Gary Ross, Tom Bryson, Daniel Blanchett, Wayne Walters (c), Daniel Nielsen, Clemente Lopez, Daniel Dyer, Steve Dogbe, Elliot Onochie, Brian Haule, James Bent. Subs: Kwasi Frempong, Jason Haniff (for Bryson), Daniel McGonigie (for Lopez), Craig Nicholson