Forest had looked destined for a 10th league game without a win, with the Canaries looking on course for the three points thanks to Jonny Howson’s first-half strike.
But Assombalonga conjured up an 85th-minute equaliser – his 10th goal of the season – and then the roof was lifted off the City Ground as Antonio also bagged his 10th of the campaign to secure a much-needed win for the Reds, while extending Norwich’s own frustrating run to one win in eight games.
It was Forest who started brightly, with Tom Ince agonisingly close to giving them the lead following good work from Antonio down the left, with the winger’s shot narrowly clearing the crossbar.
However, the home side could not make their short spell of dominance count and, when Norwich were utterly ruthless with their first opportunity of the match as Howson put them in front, the home side’s confidence drained away visibly.
Lewis Grabban could and should have doubled the Canaries’ advantage when presented with a free header at the near post following Russell Martin’s cross, but directed his effort wide of the target, albeit only by a matter of inches.
Forest were looking nervous on the ball, with the mood having changed dramatically in the City Ground. They did carve out a handful of opportunities to level before the break, with Antonio failing to properly connect with a Jack Hunt cross at the far post and Assombalonga sending a dipping 30-yard free-kick just over the bar.
But they were half-chances at best and, when the half-time whistle arrived, it was greeted with a chorus of boos from the frustrated Forest fans.
Forest boss Stuart Pearce responded by bringing on Jamie Paterson in place of Ince during the interval. However, it was Cameron Jerome who had the chance to add to Norwich’s tally, firing over the bar amid a mad scramble by the penalty spot.
Grabban then forced a smart save from Karl Darlow at his near post with a driven strike that the keeper did well to push away, with the visitors forcing a spell of pressure.
Forest finally began to find some attacking momentum but Assombalonga’s shot, following good build up around the penalty area, was easily charged down and a spectacular effort from Antonio flew a few feet over the bar.
Assombalonga had a gilt-edged chance to level when substitute Paterson picked him out at the near post, but his header was off target. Another replacement, Henri Lansbury, then picked out Antonio in an identical position, but he failed to connect with the ball completely,
But Forest did finally make the breakthrough with five minutes to play as Fryatt picked out Assombalonga eight yards out, and the striker made no mistake as he prodded the ball home.
And it got even better for Forest, as Assombalonga then turned provider deep into stoppage time, collecting a fine ball from Paterson before sending Antonio loping into the box, where the winger dispatched the winning goal into the bottom corner of the net.