Kevin Gallen came off the bench to set up Rangers' seventh away win of the season.
Gallen took over from Chris Kiwomya with six minutes remaining and his impact was immediate.
With two minutes to go, former England under-21 striker Gallen drifted past two tackles on the edge of the Portsmouth area before beating goalkeeper Russell Hoult with a fierce right foot shot.
Shocked Portsmouth let in another a minute later when Sammy Koejoe got down the left and his low centre appeared to rebound into the net off Portsmouth defender Andy Myers.
Portsmouth had fought back to level with a goal early in the second half and had several times threatened to snatch a winner.
Rangers took the lead in the 30th minute with a candidate for goal of the season, a superb 30-yard shot from Richard Langley which crashed in off the underside of the bar.
Pompey levelled in the 53rd minute when Thomas Thogersen got down the right and his centre was met at the near post by a firm strike from Rory Allen.
Allen's goal was his first since August 21 and only his third of an injury-racked season.
Only then did Rangers start to lose control of the midfield and Shaun Derry and Lee Bradbury missed chances to put the home side in front.
But at the other end Rangers always looked dangerous, with Langley shooting onto the roof of the net and Koejoe putting another effort into the side netting.
Langley, who with the more experienced Gavin Peacock ran the midfield, marred an otherwise impeccable display with a booking in the 61st minute for getting involved in a fracas between Bradbury and Peacock.
Rangers also had Paul Bruce booked for a foul and Pompey's Bradbury was also shown a yellow card for the same reason.
But with the match finely balanced the introduction of Gallen proved the key for Rangers and they swept through for a deserved victory.