Forest kept their promotion ambitions on track with their seventh win in the last nine games.
Goals by Ricky Scimeca and skipper Chris Bart-Williams in each half were good enough to see off a Portsmouth side who failed to make the most of their chances.
Pompey's frustration boiled over in the second half when midfielder Shaun Derry was sent off for two bookable offences.
Pompey had started brightly enough but fell behind after just 11 minutes when Scimeca exploited glaring spaces in the visiting defence to volley home from a cross by young Irishman Keith Foy.
Derry was inches away from an equaliser six minutes later when his left-foot drive flew wide and player-manager Steve Claridge was furious that a 26th-minute penalty decision went against him.
It seemed just a question of time before Portsmouth equalised and Luke Nightingale was furious with himself for missing a 33rd-minute chance set up by Thomas Thogersen.
Claridge also saw an effort cleared off the line by Ben Olsen and former Forest player Nigel Quashie also missed an inviting opportunity as Pompey trudged off still a goal behind at half-time.
They were made to pay in the 63rd minute when Quashie brought down young striker Andy Reid and Bart-Williams curled the free-kick beyond Russell Hoult.
The second goal did much to kill off Pompey's hopes and in the closing minute Forest could easily have doubled their lead.
Jason Crowe prevented a certain goal by blocking Reid's effort just short of the line and in the last minute Scimeca was denied a second goal when he rattled the bar.
In time added on Nightingale saw an effort come back off the bar after being put through by Quashie but that effort summed up a miserable afternoon for Claridge's side.