Thistle's 100 per cent home record in all competitions this season was ended by a first-half goal from Andy Barrowman, who later saw red as both sides finished with 10 men.
The hosts enjoyed plenty of early possession but the visitors created the first decent chance when Craig Dargo was denied by a fine save from Thistle keeper Graeme Smith.
Home defender Aaron Muirhead was then sent off on 28 for a professional foul on Dargo just outside the penalty area.
Dunfermline quickly took advantage of their extra man and took the lead in the 35th minute when Barrowman headed home from Josh Falkingham's centre.
Partick rallied after the restart and Kris Doolan headed an Aaron Sinclair cross just over, although Smith was called into action at the other end as he made a fine double save.
The home side had a glorious chance to equalise in the 70th minute when Andrew Geggan brought down Chris Erskine in the penalty area but Ross Forbes missed his spot-kick as it rattled the post.
Pars goalkeeper Paul Gallacher then denied both Erskine and Forbes in the closing stages as Dunfermline held on despite Barrowman spoiling his day by getting sent off at the death after clashing with Sinclair.