Donegal stayed unbeaten in the league with a five-point victory at Glennon Brothers Pearse Park.
The visitors made the most of Longford's indiscipline in the first half, with free-taker Lee Henderson firing seven of their nine points. The Ulstermen completely dominated the opening 25 minutes and held a seven-point cushion at one point.
Longford, however, rallied late in the closing stages and a deflected goal from Joe O'Brien and a brace of points from Eoin Donnellan and Cathal Mullane left one point between the sides at the interval.
Longford made a dream start to the second half, bagging 2-1 in five minutes. A brilliant individual goal from Mullane set the tone. The home side were then handed a piece of good luck when O'Brien scored a fortuitous goal and in doing so put them six points ahead.
Donegal refused to be pushed aside, however, and got back on track thanks to excellent free-taking from Henderson, who registered 12 points from the placed ball.
Ciaran Matthewson added two second-half points, while a close-range goal from Kevin Campbell with 10 minutes left secured the points for Donegal.
Scores – Donegal: L Henderson 0-13 (12fs, 1 '45'), K Campbell 1-0, C Matthewson (1f), S McVeigh 0-2 each, P Sheridan, D Cullen, C McDermott 0-1 each. Longford: J O'Brien 2-1, E Donnellan 0-5 (4fs), C Mullane 1-2, D Tanner 0-1.
Donegal – P Burns; M McGrath, J Boyle, M Bonnar; C McDermott, J Donnnelly, J Browne; P Sheridan, D Cullen; R McDermott, E McDErmott, S McVeigh; L Henderson, K Campbell, C Mattewson. Subs: P Hannighan for McVeigh (49), E Organ for Campbell (66)
Longford – P Cullen; R Breslin, B Stakem, K Murray; C Egan, D Tanner, R Donnellan; P Barden, C Flanagan; E Donnelan K Connelly, C Mullane; G Ghee, J O'Brien, J Casey. Subs: J Newman for Casey (44), G Moore for R Donnellan (60), C Kavanagh for E Donnellan (65)
Ref – O Elliot (Antrim)