Millwall had been in control of the game after Ben May had fired them into a fourth minute lead, with Paul Smith having to make a fine save to prevent Darren Byfield extending their lead and Paul Robinson clattering the bar with a header.
But Forest rallied after the break, with Junior Agogo pulling them level with a well taken strike and James Perch edging them ahead, with two goals in the space of three minutes.
Skipper Ian Breckin headed a third for the home side in the 83rd minute to make things comfortable for Forest, but this gave an unfair slant to the scoreline in a game in which Millwall had them on the back foot for long periods.
Forest keeper Smith had already made one important save early on to deny Alan Dunne, as he tried his luck from the edge of the box.
But the Forest keeper could do little in the fourth minute, after being left exposed by a rare slip from the normally reliable Danny Cullip, with the big defender making a hash of a simple header back to his keeper and allowing May to race through and slip a low shot into the back of the net.
Forest responded with a powerful effort from Agogo, which Lenny Pidgeley saved well, palming around his post. But it was Millwall who remained on top and who were unlucky not to add to their advantage.
They had two further chances, as Charlie Lee fired a rocket of a shot just wide of the post and Robinson shook the bar with a powerful header from a corner.
For the first time, Forest began to force some pressure of their own, with Agogo directing a Kris Commons cross narrowly over and Pidgeley somehow clawing a close-range header from Wes Morgan off the line.
Smith had to produce another vital save as Byfield raced clear of the Forest defence. But, by the half-time whistle, Forest will have felt a little unlucky to have gone in behind, as Perch, Breckin and Nicky Southall all came close.
The second half began with a determined effort from Neil Harris, but he saw his header palmed away by Pidgeley.
Although Millwall too came close to scoring as only a fine challenge from Morgan denied Byfield, as he raced to connect with a Danny Senda cross at the far post, where the slightest of touches would have guided the ball home.
The game was to change dramatically in the space of a few minutes however, as two quick goals put Forest in the driving seat.
The first was to come from Agogo, who burst through the centre of the Millwall defence, before lifting a rising shot over the advancing keeper as the visitors were caught flat footed.
And, before the celebrations died down, Forest grabbed a second as Perch connected with a Commons cross from close range and, although Robinson cleared, assistant referee Radford ruled that the ball had crossed the line.
Breckin ended any thoughts of a Millwall fightback with a powerful header to make it 3-1 in the 83rd minute.