Morton's Kevin Finlayson and Chris Millar had long-range drives saved by Sean Murdoch, but Barry Sellars blazed a free-kick narrowly wide to establish the visitors' presence.
In the 18th minute Morton went ahead when an in-swinging Andy McLaren corner eluded the grasping fingertips of Murdoch and found the top corner of the net.
Several similar kicks from McLaren troubled the Forfar defence, and Murdoch just kept the ball out as it threatened to curl in.
Morton's Stewart Greacen had a shot deflected behind after Millar nudged the ball into his path.
Forfar's Paul Lunan lifted the siege with a clever run which brought a corner, but neither goalkeeper had been called into action by half-time.
Peter Weatherson headed over a McLaren cross as 'Ton resumed their superiority but, in the 63rd minute, Greacen was sent off for a foul which was certainly cautionable but referee Smith's decision to flourish the red card seemed very unfair on a player with an unblemished reputation.
Despite the reduction in numbers, Ryan Harding headed 'Ton further ahead when he was left unmarked from a Scott McLaughlin cross.
The best goal came with a marvelous spurt from substitute Jason Walker, who gave Millar a sublime pass to enable him to beat Murdoch with a low swerving drive into the corner of the net.