Partick racked up only their second away victory of the season with a triumph over Morton.
Morton, however, should have edged in front just before the interval but Jonathan Tuffey saved from Brian Wake after he burst through from the left.
The Jags took the lead in the 53rd minute when David Rowson's cut-back was tapped into the empty net by Buchanan.
Thistle then doubled their tally three minutes later when Donnelly swept the ball past Stewart from eight yards out.
Morton: Stewart, A Trialist, MacGregor, B Trialist, Greacen, Paartalu, Finlayson (Kane 76), McGuffie, Wake (Grady 68), Weatherson, McAlister.
Subs Not Used: Harding, Monti, McWilliams.
Partick: Tuffey, Paton, Archibald, Hodge (Adams 46), Robertson, Maxwell, Cairney, Rowson, Buchanan (Lovell 65), Donnelly, Erskine (Corcoran 68).
Subs Not Used: Corrigan, Hinchcliffe.
Goals: Buchanan 53, Donnelly 56.
Att: 2,738
Ref: G Salmond
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