Slough's ranks were bolstered by the arrival of loanees Paul Stonehouse and Declan Benjamin from Oxford City, and both came straight into the starting eleven, with Dan Hicks also replacing Gary MacDonald at centre-back.
In the opening minutes, Jonah Ayunga and Charlie Davis fired long-range attempts off target, Davis going the closest, with his effort whistling just past Mark Scott's post.
Slough began to exert dominance on the game with a high tempo of play, not allowing Dorchester, missing several regular first-choice defenders on the night, no time to settle on the ball.
Dan Hicks headed over from a whipped Lewis Putman free kick, and Dorchester keeper Shane Murphy made a terrific reaction save to stop what would have been an own goal from Jack Alexander.
Dorchester made a change on 21 minutes, bringing on defender Matt Oldring, who had arrived late, for Tony Lee – but the change seemed to make little difference in the first half, and Slough took the lead just a minute later.
Murphy had a couple of moments to forget as he helped Slough on their way. A poor kick from the keeper was controlled by Warren Harris and quickly sent back low towards goal on the wet surface, with the ball squirming through the grasp of Murphy and trickling over the line. Harris celebrated his first goal since scoring in the playoff semi-final at Rugby in April 2014.
Three minutes later, Slough doubled their lead. Oldring's poor introduction to the game continued as he was penalised for a handball 25 yards out, and up stepped Putman to blast an unstoppable free kick over the wall and past Murphy.
Putman had another attempt from inside the box minutes later, but this time Murphy made a stop, and then moments later Sean Fraser's dangerous cross flew all the way across goal and just wide, with Charlie Mpi almost getting a touch at the near post.
Dorchester steadied themselves and kept Slough at bay for the remainder of the half, but the Rebels were superb to watch, working hard to keep the pressure on and force mistakes from the visitors.
Three minutes into the second half, Slough almost caught Dorchester cold when a quick throw from Stonehouse, who went on to win the man of the match award, found Mpi, but Murphy blocked his shot.
Slough's third on 59 minutes put the Rebels one step closer to victory. Again, poor defending from Dorchester left Harris with the simple task of playing the ball sideways for Charlie Strutton, who was in space ten yards out and able to apply a routine finish and claim his first Slough goal.
Dorchester rallied as Slough sat back a little, and Mark Scott made a number of saves to keep the score at 3-0. He got down well to save from a corner, and later pushed a shot from Mark Jermyn aside.
With twelve minutes left, the visitors pulled a goal back as Bradley Tarbuck was played in by substitute Nick Crittenden, and he kept his composure to round Scott and slot the ball home.
Slough brought on Adam Martin as an extra player in midfield as they looked to see the game out, and Dorchester pressed but were unable to find a way through, though Scott needed to make another good save in the third minute of time added on.
But a minute later, Mpi rounded off the scoring when he broke clear of the last defender, took the ball around Murphy and finished from a tight angle to complete a very positive night for the Rebels.
Dorchester Town Line up
Shane Murphy, Oakley Hanger, Neil Martin, Mark Jermyn, Tony Lee, Jack Alexander, Charlie Davis, Lewis Morgan, Jonah Ayunga, Ben Watson, Bradley Tarbuck. Subs: Jason Brookes, Nick Crittenden, Matt Oldring, Alan Walker-Harris.