Steve Cotterill's side almost turned in the textbook away performance against their in-form Welsh opponents but were undone by a Mason goal on 70 minutes.
The manager then gambled with a triple subsitution on 78 minutes but his men were unable to force an equaliser.
Defeat leaves the Reds just a point above the relegation zone but their display gives hope of a climb up the table over the next few weeks and Cotterill felt a draw would have been a fair result although he admitted: "We have played better than that.
"We were better in the second half than the first.
"It was a game littered with silly free-kicks and I didn't think we got many decisions from the referee."
The hard-fought nature of his side's victory delighted home boss Malky Mackay who said: "It was a battling three points and I'm pleased about that.
"Forest are a tough team who set themselves up make sure that we didn't take them out easily."