Paul Groves' two-year reign as manager of Grimsby is over following a run of 10 games without a victory.
But the 37-year-old former West Brom and Leicester midfielder will remain
on the playing staff at Blundell Park.
Graham Rodger, Groves' former assistant, has been put in charge for Saturday's game against Brighton.
The Mariners, just one point off the relegation zone, have had some torrid results this season and Sunday's 6-0 drubbing by Oldham sealed Groves' fate.
Grimsby chairman Peter Furneaux said the decision was made at a meeting with Groves on Monday morning, although sacking him was not the original intention.
"The decision wasn't up for discussion before and it came about because we needed a way forward and we weren't getting a way forward that would turn it around," said Furneaux.
"Individually we still think the players have got great ability but collectively."
Groves joined Grimsby in August 1992 and made 184 league appearances before joining West Brom in July 1997.
He returned to Blundell Park in a playing capacity in August 1997, before
taking over from Lennie Lawrence as boss in December 2001.
Despite keeping the Mariners in the First Division in his first season in
charge, Groves' fortunes nose-dived last term and the Mariners were relegated to
Injuries and a tight budget have not helped this time around, but the heavy defeat at Oldham on Sunday proved the final straw.
And the Grimsby board of
directors moved swiftly to relieve him of his duties.