Jonathan Smart doubled the home side's advantage when he slotted home from close range on 73 minutes.
And Bobby Linn completed East Fife's win two minutes from the end with a left-foot drive.
East Fife: Michael Brown, McGowan, Ovenstone, Smart, McCulloch, Young (Newbigging 90), Muir, Park, Johnstone, Hislop, Linn. Subs Not Used: Lyness, Durie, Fagan, Cargill.
Booked: Park.
Goals: Hislop 33, Smart 74, Linn 88.
Forfar: Gallacher, Ross, Bishop, Bolocheweckyj, Iain Campbell (Mowat 28), Ross Campbell, McCulloch, Sellars, Martyn Fotheringham (Watson 69), Templeman (Smith 66), Hilson. Subs Not Used: Duffy, Deasley.
Sent Off: Bishop (13). Booked: Ross, Ross Campbell.
Att: 469
Ref: Stephen Finnie
Source: BBC Sport
Source: BBC Sport