The Mariners had won five of the last eight meetings between these two sides and looked set for another win only for Bent to strike with a minute to go.
Grimsby had gone ahead with their first attack in the eighth minute - player-manager Paul Groves netting from loanee Richard Hughes' cross, with Thomas Gaardsoe unable to clear before the shot crossed the line.
Ipswich were level just four minutes later, Marcus Bent crossing for Darren Bent to dive and head home.
The game continued in an end to end fashion with John Oster having a shot blocked and debutant keeper James Pullen saving a John McDermott header before Darren Ambrose volleyed over for Ipswich.
Darren Bent glanced a header just wide form a Gaardsoe cross before Grimsby regained the lead in the 24th minute.
Michael Boulding collected a loose ball and ran to the edge of the box before having a shot deflected off John McGreal which somehow squirmed under Pullen.
Ipswich boss Joe Royle made two changes at half time - McGreal and Jim Magilton being replaced by Richard Naylor and Jermaine Wright, and Naylor should have equalised in the 59th minute.
Darren Bent's cross picked Naylor out unmarked six yards out but he guided his header well wide.
Tommy Miller then had the next great chance for Ipswich when he seized on a loose ball just inside the area but Danny Coyne dived to tip his shot round the post.
Ipswich grabbed the equaliser in the 89th minute when a corner from substitute Martijn Reuser was flicked on to the bar by Naylor. Gaardsoe then hit the post before Darren Bent bundled the ball home.
Ipswich could even have won it in stoppage time when Reuser sent in another corner which Coyne bravely punched away as Reuser closed in.