It was the first time that Newcastle had lost by such a heavy margin since the days of Graeme Souness against Fulham just under three years ago.
While Pompey will rightly take the plaudits, Newcastle were woeful and inept and got exactly what they deserved on the eve of the Tyne-Wear derby.
It was Newcastle's first home defeat of the season and, after conceding just four goals in their previous games at St James' Park, Pompey doubled that tally.
And to compound their misery, Newcastle will now head to the Stadium of Light without Nicky Butt whose first-half caution was his fifth of the season.
Sam Allardyce rang the changes to his Newcastle side showing exactly what he thought about the lack-lustre defeat at Reading.
Out went Emre, Obafemi Martins, skipper Geremi and defender Habib Beye, who did not even make the bench, while Shay Given failed a fitness test on a slight groin strain.
Steven Taylor was recalled at right-back and Alan Smith started alongside Michael Owen while Charles N'Zogbia and James Milner were in midfield along with Butt who was given the captain's armband.
After David James pulled off an excellent save to deny Smith, Pompey stunned the home crowd with a three-goal salvo within the space of just four minutes.
Each time the United defence was torn to shreds as first Noe Pamarot, then Benjani and finally John Utaka exploited some woeful defending.
Pamarot got the ball rolling after eight minutes with a 25-yard thunderbolt from a Butt clearance following a corner.
Within 60 seconds it was 2-0 when Benjani outmuscled Claudio Cacapa to curl a left-footer beyond Steve Harper and then the Brazilian defender was to blame again after 11 minutes as Utaka made it 3-0.
Effectively it was now a damage-limitation exercise for shell-shocked Newcastle as Allardyce raced from directors' box to the dugout.
An own goal threw Newcastle something of a lifeline on the quarter-hour mark when a N'Zogbia shot rebounded to Owen whose shot struck James and rebounded off Sol Campbell and into the net.
Allardyce hauled off Cacapa and replaced him with David Rozenhal after 17 minutes and the Czech defender got away with a blatant handball which was somehow missed by the over-fussy referee Chris Foy.
An early second-half strike was necessary if Newcastle were to have any hope of clawing their way back into the game, but frankly that never looked likely.
Newcastle looked like a schoolboy team as they launched aimless balls forward hoping for a break rather than creating an opening.
Martins and Emre replaced Jose Enrique and the disappointing Milner as United's powder-puff attack failed to produce any worthwhile efforts.
And it was Pompey who added insult to injury after 71 minutes when a Niko Krancjcar free-kick from the left was whipped in and missed by Taylor and ended up in the back of the net to end a miserable day for Newcastle who saw their disgruntled fans pile out of the stadium.