Two goals in 10 second-half minutes sent Millwall sliding to a second successive defeat against promotion-chasing Forest at the City Ground.
Having matched Colin Calderwood's side for the first half, and much of the second, The Lions were undone by a couple of soft goals from set-pieces.
It was a miserable afternoon all-round for The Lions, with Cheltenham's win against Brighton and Crewe's late fightback  at home to Swansea sending Kenny Jackett's side back down into the dreaded drop zone.
Boss Jackett made five changes to the side beaten by Doncaster the previous weekend, but the new-look Lions were still settling in and could have found themselves a goal down within two minutes.
Debutant Will Hoskins looked to be offside when he received the ball from Kris Commons, but was given the benefit of the doubt by the linesman and allowed to carry on his run. Thankfully the striker took an extra touch, enabling keeper Rhys Evans to gain valuable ground and parry his shot for a corner.
Seconds later the lively Commons broke down the right, but he volleyed high and wide from a good position.
Millwall recovered from a nervy start and this developed into an even, keenly fought encounter much of the time.
However it was the pace of Lions midfielder Ali Fuseini that prevented Forest taking the lead on 14 minutes. Marc Laird was caught in possession deep in his own half by Nathan Tyson, who set off like a train towards goal. But teenager Fuseini somehow made up ground and then produced a brilliantly-timed tackle to nick the ball off the front man as he prepared to shoot.
At the other end Neil Harris' deft ball through to Jay Simpson caught the home defence napping, but he seemed in two minds as to whether to pass to Gary Alexander or shoot. Simpson opted for the latter and unfortunately scuffed his shot well wide.
Having nullified Forest during the first 45 minutes, Millwall found themselves a goal behind within two minutes of the restart when an excellent corner from Commons was headed home by Bennett.
Millwall had barely time to regroup before Forest, whose second-half display was far more fluent than the first, doubled their lead. This time Commons corner was headed on by Wes Morgan and there was Luke Chambers to sidefoot home.
It could have been three just past the hour mark when another Commons' corner was met by Morgan, but this time the big defender saw his header crash against the top of the bar.
Former Lion Hoskins should have put the game beyond doubt after 65 minutes when he broke through, but with time to pick his spot, he dragged his effort well wide of Evans' left-hand post.
Boss Jackett introduced Lewis Grabben and then Dave Martin for his debut, the latter becoming the 36th different player to pull on a Millwall shirt this season.
Although The Lions livened up a bit, the damage had already been done and Forest simply shut up shop safe in the knowledge of a job well done.
For Jackett, however, he has to pick his dejected troops up now for a must-win clash on Tuesday night at home to Southend United.

Nottingham Forest: Smith, Wilson Chambers, Morgan, Commons (Perch, 81mins), Tyson, Hoskins (Holt, 68mins), Cohen, Clingan, McGugan (Davies, 84mins), Bennett. Subs not used: Breckin, Sinclair.
Bookings: Bennett (78mins), Holt (90+2mins)
Scorers: Bennett (48mins), Chambers (58mins)

Millwall: Evans, Bignot, Frampton Fuseini, Robinson, Whitbread, Brkovic (Martin, 57mins), Laird, Harris (Grabban, 66mins), Alexander, Simpson. Subs not used: Pidgeley, Senda, Forbes.
Bookings: Whitbread (30mins), Robinson (63mins), Frampton (70mins), Bignot (75mins)

Ref: C. Oliver
Att: 17,046 (away 637)