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Two first-half goals from Grimsby stunned the visitor, with Charles Vernam breaking the deadlock with a well-struck effort 22 minutes in, beating Andre Onana at his near post, before another goalkeeping blunder from a corner allowed Tyrell Warren to pounce on a loose ball eight minutes later.

Grimsby, unbeaten in its opening five games in England's fourth tier, was defensively resilient thereafter, as Ruben Amorim's side sought to reduce the arrears.

United laid siege to Grimsby's goal, and its offensive onslaught finally came to fruition when Bryan Mbeumo slotted a curling effort into the bottom corner, netting his first goal for the club, to set up a nervy ending for the hosts.

When it seemed as though the writing was on the wall for United, Harry Maguire produced the equaliser in the 89th minute, heading home from a Mason Mount corner. 

Forcing the tie to penalties, it was Grimsby that held its nerve in an excruciating 26-goal penalty shootout. Onana had saved Clarke Odour's effort, and Christy Pym replicated that on Matheus Cunha's effort to force sudden death. 

Neither side blinked until Mbeumo stepped up for his second spot-kick, smashing that one against the crossbar having converted the first. 

Winless in their first three games of the campaign, with two defeats and a draw, the pressure is now mounting on Amorim and his players to deliver, with a visit from Burnley awaiting the Red Devils on Sunday.

Elsewhere, Everton continued where it left off at the weekend, beating League One Mansfield Town 2-0, thanks to a sublime strike from Carlos Alcaraz, before an 89th-minute goal from second-half substitute Beto wrapped up the Toffees' qualification. 

Fabian Hurzeler's Brighton thrashed Oxford United 6-0 away from home, with Stefanos Tzimas netting a second-half brace.

Fulham also progressed to the third round with a 2-0 win over Bristol City.