Goals from Neil Harris - a former Forest player - and Jay Simpson had looked to have secured victory for Kenny Jackett's hosts, who climb out of the relegation zone on goal difference.
But Chris Cohen and Nathan Tyson scored within two minutes of each other to prevent Colin Calderwood's side from falling to their third defeat in a row on the road.
Jackett gave debuts to goalkeeper Rhys Evans and striker Lewis Grabban, both of whom signed for the club this week.
And Evans was busy early on, spreading himself well to block from Sammy Clingan after Luke Chambers' early throw had caused panic, before Nathan Tyson volleyed over when well-placed.
And the visitors were so unlucky on 21 minutes when Chambers headed Clingan's free-kick against the post before it hit Evans on the back of the head and was hooked to safety.
Millwall responded, and they took the lead six minutes later when Harris fired a superb free-kick into the top corner from the edge of the D after James Perch had fouled Simpson.
Harris, who rejoined Millwall from Forest 12 months ago, had scored only one league goal during his two years in Nottingham and clearly enjoyed the strike.
Just eight minutes into the second period it was 2-0 as Arsenal loanee Simpson took his time before finding the net after a corner was only half-cleared.
The New Den had found its voice and Forest looked a ragged imitation of a promotion-chasing side, although they pulled what looked like a consolation back through Cohen's close-range finish five minutes from the end.
But Tyson reacted first when a loose ball dropped in the box to send Forest's 800 travelling fans into delirium, while a valuable point was greeted like a defeat by the home fans.