Daniel Hughes missed a late free chance as Donegal made it through to the semi-finals of the Ulster championship in Ballybofey.
Hughes, set up by skipper Benny Coulter, got Paddy O'Rourke's charges off to a great start when he slotted home his first championship goal in the eighth-minute.
That strike separated the sides at half time – Down led 1-06 to 0-06 with Rory Kavanagh and Michael Doherty scoring 0-02 apiece for Donegal.
A 38th-minute goal from Kavanagh levelled matters, and Donegal finished the stronger with late points from Michael Hegarty and Adrian Sweeney securing them a semi-final shot at Derry on June 18.
Championship debutant Michael Doherty helped Donegal get up and running with two frees in the opening six minutes. Wing back Eamonn McGee also traded points with Down midfielder Dan Gordon.
Still, at 0-03 to 0-01 in front, the Tir Chonaill men were then rocked back onto their heels when Coulter, who was kept strangely quiet throughout, evaded the grasp of his marker Paddy Campbell and set Hughes up for a coolly-taken right-footed goal.
Down managed to push three points clear by the half hour mark. After the goal, the home side initially responded with overs from Conan Dunne and Kavanagh.
Yet possession was going Down's way with Gordon lording it at midfield. Three straight points from Doyle (free), Ronan Murtagh and Hughes opened up Down's three-point lead (1-05 to 0-05).
They held onto it for the break with Kavanagh and Doyle - with his second - swapping points in the first half's closing stages.
A fisted effort from Coulter just a minute into the second half enabled Down to go further clear. Despite that, in the space of a minute, Donegal were brilliantly back on terms.
First, wing back Barry Dunnion soloed forward for his first championship point and then left half-forward Kavanagh sprinted clear to rocket an unstoppable shot beyond the reach of Down keeper Brendan McVeigh. At 1-07 apiece, it was anyone's game.
The sides went point for point - Paul Murphy put Down in front before Donegal full-forward Dunne squared it up.
The tireless Hegarty and Michael Walsh exchanged scores. Then Donegal were left to rue a missed goal chance when Ciaran Bonner's effort was whacked in off the woodwork.
Donegal skipper Christy Toye mustered a 53rd-minute point. Hughes squared it up again at 1-10 to 1-10 - the seventh time the sides were on a level footing.
Donegal looked the more composed with 2003 All-Star Adrian Sweeney - in for his 31st championship outing - now on the field. Hegarty and Sweeney swung over the decisive points on 59 and 61 minutes.
Not even six minutes of injury-time could allow an increasingly desperate Down side back into it. A 69th-minute free from Doyle left the visitors a single point behind.
The Mourne men got a further chance, in the dying seconds, when Galway referee Gearoid O Conamha awarded them a free out on the right wing but with regular free-taker Doyle subbed off, top scorer Hughes' gallant attempt sailed to the right and wide, and with that, the Donegal celebrations began.
Donegal: P Durcan; N McGee, P Campbell, K Lacey; E McGee, M Hegarty, B Dunnion; N Gallagher, B Monaghan; C Toye (capt), C Bonner, R Kavanagh; C Kelly, C Dunne, M Doherty.
Down: B McVeigh; M Cole, B Grant, A Molloy; P Murphy, J Clarke, A Scullion; A Rogers, D Gordon; E McCartan, L Doyle, R Murtagh; D Hughes, B Coulter (capt), M Walsh.
Referee: Gearoid O Conamha (Galway)