Mariners keeper Phil Barnes then made a fantastic save to keep out Dean Lewington's header when the full-back was left unmarked six yards out.
Both sides had other penalty claims turned down as the first half remained goalless - notably when Grimsby defender Nick Fenton appeared to handle a cross.
But in a crucial few seconds of action, Cameron's curling shot hit the post, and with at least one Dons player appearing to stand in an offside position, the assistant referee's flag was raised.
But Joslin pointed to the spot after adjudging that Nick Hegarty's subsequent challenge on Danny Swailes deserved a penalty.
If Grimsby's hearts were broken by that decision, they were well and truly shattered when O'Hanlon's header, which went in off Rob Atkinson, made it certain that Paul Ince's side - with captain Andrews leading from the front - would be collecting their first trophy.
Milton Keynes Dons manager Paul Ince: "It's my first trophy as a manager, but it's been a long, hard road - we've played some hard teams to get here, but we feel we can be a handful for anybody.
"We created a lot of chances, and it was just a case of waiting for one to go in, but we seem to be good at scoring penalties in this competition."
Grimsby boss Alan Buckley: "It was a game of two penalties - ours looked more of a penalty than theirs did, and our players are up in arms as we felt a decision [the offside flag] had been given in our favour - but the referee didn't see it that way.
"But we've got no complaints after 90 minutes. They're a decent team and pass it well, but goals change games - we had a golden opportunity with our penalty but didn't take it."
Grimsby: Barnes, Clarke, Atkinson, Fenton, Newey, Hegarty, Hunt (Toner 79), Bolland, Boshell, Till (Jones 61), North (Bore 46). Subs Not Used: Montgomery, Bennett.
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