The Lions had not won in eight attempts but Fuller’s poacher’s instincts saw him pounce on a poor header from Stephen McLaughlin seven minutes from time and snatch a 1-0 win at the City Ground.
It saw Forest extend their own recent run to just one win in 11 outings – a success at Derby which helped halt speculation about Stuart Pearce’s future, but it will begin again if the former defender does not inspire a change in form quickly.
Forest had not scored in the opening 30 minutes of a game since September, and they are only rarely looked like ending that run against the Lions, during an insipid first half.
The closest they came was in the first five minutes, when Henri Lansbury drove a shot towards goal, following a huge long throw from Michail Antonio, but saw it deflected over the bar.
A Millwall free-kick from Shaun Cummings was the source of Forest’s clearest opening, as he attempted to fire the ball back towards his own goal, but managed to send Antonio racing clear.
Cummings’ blushes were spared by goalkeeper David Forde, who was quick off his line to make a vital, brave save.
Millwall were forced to make a change as Lee Martin collected an injury, with Ed Upson coming on in the 15th minute.
Dorus de Vries, who is in negotiations over a new contract at the City Ground, demonstrated why as he made an outstanding save to deny the visitors, brilliantly pushing away a Mark Beevers header, after Upson had fired in a free-kick towards the near post.
Stephen McLaughlin saw a shot charged down, with Forde again alert to save when Robert Tesche headed the rebound towards goal.
Henri Lansbury almost added to the two goals he scored at Fulham in Forest’s last outing, but his free-kick skimmed the roof of the net as it went over.
Forest were denied by the bar twice in the opening 10 minutes of the second half.
Tesche saw a spectacular 30-yard effort bounce away off the woodwork, with Chris Burke’s header from the rebound cleared by Beevers.
The second effort saw Forest come even closer after Antonio had crossed dangerously, with Forde seeming to get the slightest of touches onto a powerful header from Assombalonga, to send the ball bouncing away off the bar for a second time.
Substitute Lee Gregory looked as though he might snatch a goal for Millwall, as he battled through three Forest players and into the box, before Lansbury finally got in a tackle as he shaped to shoot.
But it was another man off the bench who had the decisive impact, as Fuller pounced on a weak attempt from McLaughlin’s header back to De Vries to beat the keeper to the ball and prod into an empty net in the 83rd minute.