Oliver Lancashire was red-carded when he handled on the line, and Phil Edwards dispatched the 67th-minute penalty. But Conlon nodded home to end a run of five consecutive defeats for Grimsby.
Stanley went ahead went King beat the offside trap and raced onto a through ball from Luke Joyce and fired past Nick Colgan.
But Grimsby were soon level. A Peter Sweeney free kick from the left was headed against the bar by Jean-Louis Akpa Akpro, and Forbes lashed home the rebound from close range.
Then a Michael Symes shot was handled on the line by Lancashire, who was dismissed, and Edwards hammered the penalty past Colgan. And with only seconds left, Bradley Wood hoisted in a free kick from the left, and substitute Conlon rose to head over Ian Dunbavin.