Reds boss Colin Calderwood had found himself under pressure following four games without a win.
But set-piece goals from Julian Bennett and Luke Chambers edged them to three points against the struggling Lions - and ended talk of a post-match protest from Forest fans.
It could have been worse for the visitors, with Wes Morgan hitting the bar and Kris Commons having an effort cleared off the line.
New signing Will Hoskins could have made an immediate impact for Forest, against his former club, after being sent clear by Lewis McGugan, but his immediate 12-yard drive was blocked by the outstretched legs of keeper Rhys Evans, with the ball deflecting past the post.
Commons ensured that Forest created more chances than they had during 90 minutes at Bournemouth last weekend within the space of the first five minutes, as he latched onto a raking pass from Sammy Clingan to lash a rising volley narrowly over the bar.
Millwall responded as Marc Laird was given room to connect with a low, bouncing effort from the edge of the box, which Paul Smith got his body behind with some swift footwork.
McGugan almost broke the deadlock with a forceful run across the face of the box that culminated with crisply struck effort that Evans struggled to block, with the keeper recovering well to claim the loose ball at the second attempt.
The boos that had rather harshly accompanied Forest off the pitch at the interval were soon drowned out as, two minutes after the restart, the home side took the lead they deserved.
A Commons corner was whipped into the middle, where Bennett rose to thunder an unstoppable header into the back of the net from ten yards, with Evans unable to save despite getting a hand to the ball.
The goal finally seemed to lift the tension around the ground, with Forest quick to double their advantage, following another spell of pressure. Commons had come close with a thundering shot that was deflected for a corner, following a clever Nathan Tyson cut-back.
The winger again provided a teasing delivery into the box, this time for Wes Morgan whose downward header caused chaos, allowing Chambers to stroke the loose ball accurately through a melee of players and into the back of the net from 12 yards.
It was clear Millwall were struggling to contain Forest at set-pieces, with Morgan powering a header against the bar from yet another Commons corner and Bennett sending a header over following a Clingan free-kick.
Hoskins fired inches wide following fine build-up play between Commons and Tyson, as Forest ran riot and substitute Grant Holt, with his first touch, sent a header flashing wide.