Down’s first Ulster Championship victory for four years saw them advanced to a semi-final meeting with either Cavan or Monaghan on 24 June.
Armagh’s much vaunted forward line failed to fire as Down edged home 0-15 to 2-7 at Pairc Esler, with Orchard boss Keiran McGeeney forced to watch on from the stand as he serves a 12-week touchline ban.
Armagh led 2-4 to 0-9 at half time thanks to two first half goals from Andrew Murnin and Mark Shields, but a shift in the balance of power saw Down rack up seven out of eight points either side of the break.
Down, starting their Ulster championship campaign at home in Newry for the first time in 18 years, went on to control the second half.
Down registered ten different scorers by the end, with Darragh O’Hanlon 0-4 (2f), Caolan Mooney, Connaire Harrison (0-2 each) leading the way.
In contrast, Armagh managed just three points in the second half and the last of them, from Jamie Clarke, came in the 82nd minute.
“It’s not enjoyable. It’s pressure, it’s pressure, and I’m just happy to get out the right side of the line,” said Down manager Eamonn Burns.
“It’s a derby game. Somebody is going to win and somebody is going to lose. It could have gone either way. It was me today, it could have been Kieran McGeeney very easily.
“We’re delighted. We took the opportunities that presented themselves and when we had to defend, we defended really, really well and really hard and really disciplined, and I’m delighted for the players.”
Armagh assistant manager John Toal, said: “Even the first 10-15 minutes they ran up a four or five point lead before we really started, we were very slow coming out of the blocks. Then we weathered the storm and came into the game well.”
“At the start of the second half we didn’t take off again, they got the lead. We had plenty of opportunities, simple opportunities, to put the ball over the bar but we didn’t take them. Gradually we just ran out of time.”