Nick Sion and Paul van Gelder had put Glenthorne 2-0 up before Marshside goalkeeper Barry Gordon saved a penalty to inspire the comeback.
Goals from Justin Woolf, two from Zohar Sharifia and a long-range effort from the appropriately named Ben Shooter, on his debut, secured Marshside's first win of the season.
Adam Brand scored for the second week running as MLFC Lions moved into top spot with a 3-0 win over Hendon Harriers. Arthur Duke and Pete Lazard were the other scorers as Lions maintained their 100 per cent record.
MLFC Brady missed out on the chance to extend their lead at the top of the table as they played out a rare goalless draw with EHRS Stonegrove.
