There was a late change to the Rebels line-up as George Wells was forced to pull out of the warmup after sustaining a foot injury at work earlier in the day, so Nathan Smart was drafted in.
In the early exchanges, James Dobson fired wide and Manny Williams headed over, but it took the Rebels just eight minutes to make the breakthrough.
Guy Hollis got in a shot which a Dorchester defender could only clear as far as Williams six yards out, and he lashed the ball home with ease.
Warren Harris hit a post as Slough pushed for a second, but the game settled down after a quick start from the hosts.
The lead was almost doubled on 40 minutes when Warren Harris fired the rebound just over the bar after Ben Harris had been denied by experienced goalkeeper Chris Weale.
Lench then fired over the bar but was more successful on the stroke of half time as he fired Slough into a two goal lead. Dorchester failed to clear a deep cross from Ben Harris, and Lench volleyed the ball back into the goalmouth, where it evaded Weale and ended up in the back of the net.
Slough missed chances to put the game to bed early in the second half. Ben Harris volleyed wide from Williams’ clever pass over the top, and the same players combined again soon after, this time Harris sliding a ball across goal which Williams could not quite reach as he slid in at the far post.
Ben Harris was denied a penalty as his shirt was tugged in the box, and his replacement Chris Flood also saw a strong appeal waved away after he was impeded.
Dorchester rarely looked a threat and Jack Turner made his first save on 70 minutes, a comfortable hold from Sam Lanahan followed by another routine low save minutes later.
Late goals have cost Slough dearly in recent weeks, but this time the Rebels held on to their lead comfortably and could have extended it further with Weale tipping over a header from Flood and Lench also denied by the keeper in stoppage time.
Dorchester Town Line up
Chris Weale, Kyle Egan, Neil Martin, Callum Rose, Ross Carmichael, Gary Bowles (c), Luke Winsper, Billy Lowes, Archie Collins, Sam Lanahan, Cameron Ebbutt. Substitutes: Ed Bastick, David Jerrard, Tom Blair, Dom Panesar-Dower, Chris Clarke.