Lee Angol’s late penalty earned Leyton Orient a share of the spoils at Brisbane ROad on Saturday.
Defender Josh Coulson returned to the starting line-up for Ross Embleton's first match as head coach, following his recent absence, while striker Conor Wilkinson returned from injury for the contest.
And the hosts got the first chance of the match as they sealed an early corner which was met by defender Dan Happe, who nodded wide.
In the 12th minute the O's went close once again as striker Wilkinson got an important touch in a crowded box but his effort was cleared off the line.
They then had a flurry of corners before Grimsby eventually cleared the ball.
Shortly after Orient defender Jamie Turley was taken off through injury after a heavy tackle and was replaced by Myles Judd in the 25th minute.
Orient winger James Brophy then had to pull off some great defending as he cleared the ball away from danger and drew a foul from Max Wright.
In the 32nd minute the visitors pumped the ball into the area a couple of times but Sam Sargeant eventually collected and started an attack for his team.
Eight minutes later Grimsby had a chance to get the ball in the box from a free-kick out on the left, but Wilkinson headed half clear, before Elliott Whitehouse scuffed it wide.
One minute later Luke Hendrie broke down the right, but his cross was blocked, and came back off him for a goal kick.
Whitehouse fired wide for Grimsby four minutes after the restart, while Hessenthaler stung the palms of Sargeant.
Max Wright sliced over as the visitors remained on top, but O's saw Jordan Maguire-Drew volley over on 57 minutes, after being found by Wilkinson.
Hector Kyprianou volleyed high and wide just past the hour mark, before making way for Craig Clay, and Wilkinson's header was somehow kept out by the feet of McKeown.
Ludvig Ohmen fired over from a Grimsby corner, while Marsh missed the target from range for the hosts.
But the deadlock was broken on 74 minutes when Billy Clarke lifted the ball over Sargeant.
O's rallied and saw the ball cleared off the line from a corner, before Marsh made way for Matt Harrold.
And the home side kept pushing in the closing stages, before finally getting their reward in the first minute of stoppage time when Clay was bundled over in the box.
Angol kept his cool to fire the spot-kick into the top left corner of the net and Urchins went close in the fourth minute of injury time when Wilkinson met Judd's cross but headed over.
Leyton Orient: Sargeant, Marsh (Harrold 81), Coulson, Turley (Judd 25), Happe, Wright, Kyprianou (Clay 65), Brophy, Maguire-Drew, Angol, Wilkinson.
Unused subs: Janata, Ekpiteta, Dayton, Sotiriou.
Attendance: 6,248 (including 1,276 Grimsby fans).
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