It was a depleted Scunthorpe side as Lee Ridley, Jim Goodwin, Ritchie Ryan, Ian Baraclough and Peter Beagrie were all missing through injury.
Brian Laws' side enjoyed the better of the game and Cleveland Taylor saw a
fizzing shot turned over the bar before having a goal ruled out for offside.
Grimsby had chances of their own but Michael Reddy was unable to capitalise
after out muscling Andy Butler, before Andy Keogh and Tommy Johnson again went close for the Iron.
Andy Crosby, Andy Parton, Tommy Johnson and Matty Sparrow all converted their spot-kicks for United.
Lee Fowler and Andy Parkinson missed for the Cods.
Iron line-up: Musselwhite, Byrne, Williams, Butler, Crosby, Taylor, Hinds, Sparrow, Corden, Keogh, Torpey. Subs: Johnson, Stanton, Rankine, Parton, Capp
Cods line-up: Brown, McDermott, Newey, Ramsden, Crane, Jones, Bolland, Barwick, Parkinson, Gritton, Reddy. Subs: Lukic, Whittle, Toner, Cohen.
The Iron's next pre-season match is against Brigg Town at the Hawthornes on Saturday, 3pm k/o.