Danny Burnell's first half goal was added to in the second by a Garry Sippetts brace after Marlow's Stuart Maclellan had been shown red.
Marlow’s Jermaine Roche had the best chance early on, but volleyed wide from a difficult angle.
After this though, Slough settled into the game and began to dominate. Garry Sippetts, drafted in to replace the injured Steve Sinclair in a three-man attack, saw a header from a corner cleared off the line by Jon Case.
Michael Watkins, so impressive against Slough in recent weeks, was kept busy with efforts from Sippetts and a Burnell shot which took a wild deflection off Case. But the Rebels were so nearly caught out at the other end, with Aaron Couch getting the better of Sweeney but blazing high and wide from a difficult angle.
Moments later, Ricky Perks was called upon to make a fine double save from Roche and Daniel Stone, before Stone miskicked in front of goal.
But soon after this, Slough took a deserved lead as Danny Burnell was played one on one with Watkins, the striker finishing confidently to register his third goal of the tie.
Slough continued to threaten and were unlucky not to double their lead before the interval. Stuart Swift tried his luck with a volley from fully 30 yards out, but Watkins got fingertips to the ball to divert it wide of goal.
With Slough looking a lot more solid defensively, Steve Bateman’s side rarely looked like conceding in the second half; but were denied by Watkins yet again on a number of occasions, with Burnell having a drilled effort turned round the post.
Marlow had recently lost both Johnny Gray and Mark Avery to injury, and after yet another left-back, Craig Roberts, was forced off just before half-time, the home side’s luck worsened with the dismissal of midfielder Stuart Maclellan for swearing at the referee after suffering a heavy Sean Sonner tackle.
But in truth, Slough looked likely to score again regardless, and did so through Garry Sippetts on 67 minutes. After earlier being denied by Watkins when one-on-one, Sippetts’ fantastic looping header from Sonner’s cross was out of the keeper’s reach and all but confirmed Slough’s progress.
With Marlow reduced to nine men late on due to an injury to Johnny Isaac, Slough pressed for a third goal and found it a minute from time. Substitute Sam Bateman played a slide-rule pass through for Sippetts, and this time the experienced striker slid the ball low past Watkins from 15 yards to confirm a comfortable win.
Marlow Line up
Watkins, Case, Roberts (Harman), Willment, Lane, Isaac, Mason (c), Maclellan, Stone, Roche (Essex), Couch (Mealing). Subs not used: Dickens, Collis.