A headed goal from Glory substitute Jamie Maclaren 12 minutes from the end seemed to be enough for the visitor to secure a barely-deserved victory until Montgomery's stoppage time interception in Gosford.
Despite aiming to regain top spot on the A-League ladder, Glory looked anything but a title contender and struggled to find fluency with the Mariners dominating possession and looking far more dangerous going forward.
Both sides hit the post in an opening half that was full of enterprise, if lacking in quality and execution.
Mariners attacker Nick Fitzgerald twice came close, firstly squirting a shot inches over the bar and then hitting the post after pouncing on a Dino Djulbic defensive error.
The Mariners comfortably had more time on the ball but Glory was not without chances to break the deadlock, most notably a sweet Nebojsa Marinkovic strike was impressively tipped onto the crossbar by goalkeeper Liam Reddy.
Glory's creative players were rarely involved, with striker Andy Keogh invariably left to plough a lone furrow up front.
It was the Mariners that looked most dangerous in general play with their movement and pace in attack.
But it was Glory who took the lead with a set-piece as Maclaren perfectly deflected a near-post header past Reddy from a Marinkovic corner.
But Montgomery ensured the spoils were shared with his net-bulging 25-metre strike in the second of four added minutes.
