Victory for Airdrie eased their relegation concerns and dealt a blow to Forfar's promotion hopes.
Forfar keeper Scott Gallacher turned a Graeme Owens cross into his own net to put the home side in front.
Paul McQuade's 25-yard effort equalised but Airdrie restored their lead through Ryan McCord's penalty after Owens was brought down in the box.
Ryan Donnelly sealed the win just before the break when he headed home Gary Mackay-Steven's cross.
Airdrie Utd: Ridgers, Bain, Owens (Blackley 77), Stallard, Smith, Johnston, McCord, Devlin, Mackay-Steven (Grant 89), Gemmill (Morton 60), Donnelly.Subs Not Used: Szpilcynski, Gray.
Booked: Bain, Mackay-Steven.
Goals: Gallacher 22 og, McCord 40 pen, Donnelly 43.
Forfar: Gallacher, Ross, Bolocheweckyj, McCulloch, Iain Campbell, Ross Campbell, Sellars (Templeman 70), Mowat, Dow (Gibson 46), Hilson (Deasley 46), McQuade.Subs Not Used: Duffy, Brady.
Booked: Mowat, Bolocheweckyj.
Goals: McQuade 31.
Ref: Des Roache (Scotland).