Now unbeaten in five league and cup wins, Sibierski grabbed the all-important match-winning goal for Newcastle after 69 minutes.
And a first Premiership goal in a black and white shirt was nothing more than his Newcastle side deserved as they followed up their recent 3-0 Carling Cup win over high-flying Portsmouth.
But for Pompey's England goalkeeper David James this could easily have turned into a rout as the pace of Newcastle caused problems throughout.
Sibierski has now struck five goals for Newcastle - not bad as the Frenchman's arrival from Manchester City was greeted by derision as he was not the big-name capture than the fans wanted.
But Newcastle's win came at a cost as James Milner, skipper Scott Parker and Charles N'Zogbia all limped out of action.
Also, the magnificent Kieron Dyer battled on despite picking up a nasty looking gash in his leg after colliding with Sol Campbell and the advertising hoardings.
St James' Park has never been a happy hunting ground for Pompey, but flying high in fourth position they must have fancied their chances against a Newcastle side languishing fourth from bottom.
Pompey started off brightly and Nobby Solano, filling in at right-back, cleared off the line from Nwankwo Kanu from a counter attack.
Another good chance looked to have fallen to Gary O'Neil who got in his shot, only for Peter Ramage to throw himself at the ball and get it away for a corner.
Parker curled an effort just wide before a poor offside decision robbed Obafemi Martins of a goal after being put clear by Dyer after 17 minutes - seven minutes before Parker left the park with a rib injury.
Pompey were fortunate when a low, drilled Milner cross was hacked by Linvoy Primus straight at Pedro Mendes and James had to claw the ball over the bar to keep out the rebound.
Milner then left the field on the stroke of half-time after picking up a back injury and was replaced by Sibierski.
The Frenchman gave Newcastle some aerial prominence in what was a one-sided second half which saw Martins fire over ferociously after 57 minutes and Sibierski denied soon afterwards by James.
But United's lightening pace up front was finally rewarded after 69 minutes when Martins surged through the middle to find N'Zogbia on the left and from his cross Sibierski turned the ball into the net.
The chances continued to pour on the Pompey goal as James denied Dyer, Sibierski twice and then pulled of a magnificent save to keep out an Emre curler.