The opening 30 minutes of the game were easily forgettable, but it exploded into life as it approached the half-hour mark when Grimsby took the lead through Martin Butler.
Butler was left unmarked at the far post and a close-range shot through keeper David Stockdale's legs put the Mariners in front and left the Darlington faithful wondering if their promotion dreams were now shattered.
Wembley-bound Town were in dreamland in the 30th minute when Butler notched his second goal.
The Darlington defence was nowhere to be seen when the big striker collected a long clearance and he turned marker Ian Miller before racing to the edge of the area and firing beyond Stockdale's despairing dive.
The home side were now desperate and in need of some form of inspiration. It came in the 33rd minute when Pawel Abbott saw Mariners keeper Phil Barnes off his line and the former Swansea striker coolly lobbed in from the edge of the area.
Seconds before the interval Abbott ghosted in at the far post to notch an equaliser as he side-footed the ball in from close range.
Two minutes after the restart Paul Bolland fired against an upright and, with the goal at his mercy, Jamie Clarke put the rebound wide with keeper Stockdale stranded.
In the 57th minute Tom Newey set up Danny Boshell but his shot was expertly turned away by Stockdale.
The winning goal came in the 64th minute from home striker Tommy Wright. He picked up possession at the edge of the area and a quick turn and shot saw the ball fly into the far corner of the net with Barnes stranded.