Mayo looked to be on the brink of a sensational upset when Brian Hunt fired them in front going into the closing stages, but Down rallied to snatch victory from the jaws of defeat.
The Mourne men looked to be well in command at half-time, leading by 0-11 to 0-7 with Paul Brannif, Paul Sheehan and Daniel Toner doing the damage on the scoreboard.
However, Mayo slowly began to fashion a mini-revival and points from top scorer Kenny Feeney, and Darren McTigue kept their side in touch at the break.
The second-half was a much more keenly-contested affair and a wonderful score from Mayo captain, Keith Higgins, levelled the game.
Murt Connolly’s side then took the lead for the first time but Down regained their composure just in time to save the day.
Scorers for Mayo: K Feeney 0-10 (8fs), K Higgins, D McTighe 0-2 each, C Freeman, P Connell, B Hunt 0-1 each.
Scorers for Down: P Brannif 0-8 (6fs, 1 65), P Sheehan 0-3, A O’Prey, D Toner 0-2 each, F Conway, M Ennis, C Mageean, M Turley 0-1 each.
MAYO: D O’Brien; S Morley, A Connolly, A Brennan; G Henry, D McConn, F Lyons; K Higgins, P O’Grady; C Freeman, D McDonnell, B Hunt; D McTighe, K Feeney, C Charlton.
Subs: P Connell for G Henry (27), P O’Flynn for P O’Grady (34), S Hoban for C Freeman (55), C McCrudden for B Hunt (65), S Markham for C Charlton (69).
DOWN: S Keith; J Smyth, M Hughes, S Ennis; F Conway, C Woods, M Cunningham; A Savage, M Ennis; P Sheehan, P Brannif, A O’Prey; D Toner, D Hughes, C Mageean.
Sub: M Turley for D Hughes (59).
Referee: S Hourigan (Limerick).