Clear-cut chances were at a premium on Saturday but substitute Mason headed home Peter Whittingham's right-wing cross from 12 yards midway through the second half.
The win extended Cardiff's unbeaten run in the Championship to seven games.
Cardiff boss Malky Mackay made one change from the side that drew at Coventry in midweek with Stephen McPhail replacing Filip Kiss in midfield. Forest were unchanged for the third successive league game.
Kenny Miller shot over the crossbar from 12 yards out in the ninth minute after being teed-up by McPhail.
It was to be one of many efforts during the afternoon to end up in the stands as both sides struggled to find their shooting boots.
Marcus Tudgay got a shot on target midway through the first half but it was a routine save for David Marshall in the Cardiff goal.
Craig Conway shot over the bar and had a header easily saved by Forest goalkeeper Lee Camp either side of the half-hour mark.
At the other end, Robbie Findley skied a shot over from the edge of the area.
Aron Gunnarsson headed wide and Don Cowie cleared the bar as Cardiff threatened just before half-time.
A speculative long-range effort from Kevin McNaughton 10 minutes after the restart did not trouble Camp.
Cardiff eventually made the breakthrough after 70 minutes when Mason, a 58th-minute replacement for Conway, headed home his fourth goal in his last six outings.
Whittingham attempted to continue his rich vein of scoring form after finding the net in each of the last four games, but Camp caught his 25-yarder.
Forest sub Paul Anderson had two chances to rescue a late point for the visitors but the midfielder headed wide from David McGoldrick's cross and then did the same from Andy Reid's corner.
