Slabber drew Grays level 10 minutes before the break
Grays Athletic came from a goal down as high-flying Mansfield were left to rue a host of missed chances.
Gary Mills twice went close with long-range efforts before Jake Speight deservedly headed the Stags ahead from a tight angle following a Mills corner.
The home side drew level through Jamie Slabber's close-range header.
But the Stags continued to dominate and Luke Jones spurned two headed chances while home keeper Lee Butcher made fine saves from Scott Garner and Speight.
Grays Athletic manager Gary Phillips told BBC Essex:
"I'm very pleased with the performance.
"Again we've had four or five debutants again today and they've put in a proper performance.
"We always look like scoring goals, but obviously we're still playing catch-up with our fitness at the moment and it shows."
Mansfield Town manager David Holdsworth told BBC Radio Notttingham:
"We had enough chances there to win three games. We got in an unbelievable amount of positions.
"We had so many chances inside their box, you wonder how it slipped away.
"You've got to take your chances in football. You've got to have that ability to put the ball in the net."
Grays Athletic: Butcher, Davis, Hoyte, Uddin, Obersteller, Graham (Taylor 71), Vernazza, Cutler, Poole (Beavan 86), Slabber, Charge.Subs Not Used: Edwards, Gaughran, Harvey.
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