William Keane scored four times as the Reds came from three down to clinch the title.
Warren Joyce’s side approached the game knowing that three points would be enough to give them the championship ahead of Liverpool, the only other side remaining in the chase.
But with half an hour played, Newcastle looked like the champions and Man Utd the side going through a horror patch of results.
Bradden Inman had given Newcastle the lead in the 16th minute and he was soon followed on to the scoresheet by Philip Airey, from the penalty spot.
Then Dan Taylor made it 3-0 to Newcastle, but with ten minutes of the half to play Man Utd began their astonishing fightback with two Keane goals inside 60 seconds.
Nine minutes after the break Keane claimed his hat-trick.
The away side took complete control and in the 64th minute skipper Davide Petrucci put his side in front for the first time.
Further goals from Keane and Ritchie De Laet made it an amazing score of 6-3.
Newcastle reserve team coach Willie Donachie: “We just couldn’t handle their pace and power in the end.
“No matter how many people we had back we weren’t quick or strong enough to stop them.
“The first half was OK, and the lads gave everything they had. To be 3-0 up was good for us.
“But the bottom line is we have players playing out of position, we’ve got first years playing, and there’s a bit of inexperience out there in terms of position and age.
“It will take its toll sooner or later.
“We started a bit nervously because it was Man United” he said.
“That disappointed me a bit because you can’t respect people too much.
“Then once we got the goals we relaxed too much. We thought we’d won it.
“Now the younger ones can now see that the level they are aiming for is high.
“Physically it’s high and you have to be strong and quick – you can’t play without it.”
NEWCASTLE RESERVES: Alnwick, Nzuzi, Moyo, Logan, J.Henderson, Folan (Nicholson 68), Maddison (Kitchen 55), Richardson, Airey, D.Taylor, Inman (McGorrigan 68). Not used: Mitchell, J.Smith.
MAN UTD RESERVES: Johnstone, Vermijl, M.Keane, Fryers, De Laet (Fornasier 79), Cole (Thorpe 51), Tunnicliffe, Petrucci, Pogba, Lingard (Blackett 87), W.Keane. Not used: Coll, Giverin.