High-flying Town had the better of the opening stages with former Sunderland striker Michael Reddy causing problems for the home defence.
Reddy was quickly into the action when setting up Paul Bolland in the fourth minute after a fine run down the right but he wasted the chance with a shot from the edge of the box that flew high over the bar.
Grimsby continued to press forward and could have taken the lead in the ninth minute when the Darlington defence only half cleared a Tom Newey corner and Gary Jones picked up the loose ball before sending a low shot narrowly wide.
The home side took their time to settle but they went close on the half-hour mark when loan signing Kyle Lafferty cut inside before firing just past the post with keeper Steve Mildenhall struggling.
There was a booking on 40 minutes for home defender Ryan Valentine after a late lunge on Newey.
Darlington lost skipper Matthew Clarke seconds before he break when, after taking a knock to the head, he was replaced by David McGurk.
In the second half the Grimsby goal was threatened in the 52nd minute when McGurk headed wide at the far post following a deep cross from Valentine.
The home side threatened again on the hour mark when Valentine picked up the ball 20 yards out but his shot was straight at Mildenhall.
In the 67th minute Darlington made a change when Paul Hopkins was replaced by Simon Johnson and then another change came on 74 when Carlos Logan came on for Matty Appleby.
The changes failed to ignite the home side and the visitors finished the strongest.
Eight minutes from time Town skipper Robert Jones almost grabbed the winner when he connected with a Newey corner but his header struck the top of the bar.