The striker, on-loan from Millwall, seized on a poorly-executed backpass from Dan Butler and coolly slotted the ball past goalkeeper Simon Eastwood from 12 yards out for his first league goal in Orient colours.
The win keeps their faint play-off hopes alive but it was a far from convincing performance.
In as lively encounter, the visitors had the better of the first-half exchanges and Jed Wallace twice came close to scoring.
Within three minutes of the kick-off, he sent in a shot that O's keeper Jamie Jones grabbed at the second attempt. Wallace then saw his shot from 10 yards out saved by Jones' foot.
Orient's only chance of the opening period fell to Lee Cook six minutes before the break. His shot from 20 yards had Eastwood at full stretch to deny the winger.
After falling behind Pompey kept pushing forward but against a resilient home defence, the nearest they came was through David Connolly.
The striker angled his shot across Jones but substitute Elliot Omozusi was well-placed to clear the ball over the crossbar.
