Wallace and Westcarr on target at Fratton Park
Jack Whatmough, Danny Hollands and Miles Storey dropped to the bench, but Adam Webster missed out completely through suspension.
Ben Chorley returned to skipper the side, while there were also starts for Joe Devera, Ricky Holmes and Westcarr.
Andy Awford made four changes from the side that beat Cambridge at the weekend.
Paul Jones was a virtual spectator at the other end, as the Cobblers failed to breach Pompey’s impregnable three-man defence.
Wallace opened the scoring with a confident finish midway through the first half before Westcarr bagged his maiden Blues goal to give the hosts a comfortable lead at the break.
Pompey continued their impressive start to the season as Jed Wallace and Craig Westcarr found the target to end Northampton’s unbeaten run.
Pompey 2 Northampton 0 Half Time:
The visitors then came close to reducing the deficit late in the half, but Devera made a brave block to ensure the hosts had a dominant interval lead.
But WESTCARR did double the advantage on 39 minutes, racing onto Nigel Atangana’s pass and blasting an unstoppable shot past Duke.
Wallace then tried to tee-up Westcarr for a simple tap-in, only for Lee Collins to slide in and make a vital interception.
Pompey then started to move through the gears and Holmes was next to go close when he burst free down the right and smashed a shot straight at Cobblers keeper Matt Duke.
The goal owed a lot to the fine work of Nicky Shorey down the left, as the wing-back collected Chorley’s pass, beat Daniel Alfei and cut back for Wallace to drive home.
Instead it was the Blues who broke the deadlock on 26 minutes, as WALLACE doubled his tally for the season.
The visitors were first to seriously threaten, but Ricky Ravenhill headed wide from Evan Horwood’s excellent cross.
But that was about the extent of the action in a largely lifeless opening 20 minutes at Fratton Park.
Ryan Taylor could not quite connect with a Holmes corner, while Ivan Toney blazed over after Paul Robinson had lost possession.
Neither side came roaring out of the traps from kick-off as the game meandered along in the opening stages.
John-Joe O’Toole finally tested Jones on 70 minutes, but the keeper got down quickly to his left to smother the shot.
It was referee Gavin Ward getting the most notice as he booked Ravenhill for handball and Toney for a foul on Holmes.
Northampton did have a couple of half-chances, but nothing to trouble a largely redundant Paul Jones in the Blues goalmouth.
There was not too much noteworthy action after the restart, although that was not going to worry a comfortable Pompey side.
Awford then made his first substitution, as the hard-working Holmes made way for Hollands.
Pompey had not looked particularly threatening in the second half, although Wallace did send in a dangerous cross that was just out of reach for the leaping Westcarr.
The speedy striker burst forward before teeing-up Atangana, whose curling shot clipped the outside of the post.
Westcarr left the pitch to a rousing reception on 77 minutes and his replacement Storey soon made an impact.
And the Frenchman then returned the favour, but Storey could only nod his chipped pass wide from close range late on.
Jones; Devera, Robinson, Chorley (c); Holmes (Hollands 72), Atangana, Dunne, Wallace, Shorey; Westcarr (Storey 77), Taylor (Butler 90+2) Pompey (3-5-2):
Duke; Alfei (Moyo 86), Diamond, Collins (c), Robertson; Mohamed (Hackett 64), Ravenhill, O’Toole, Horwood (D’Ath 46); Toney, Richards Northampton (4-4-2):
Brezovan, Whatmough, Awford, Barcham Subs not used:
Wallace 26, Westcarr 39 Goals:) 15,004 (366 away fans
Gavin Ward Referee:
Archer, Nicholls, Tozer, Sinclair Subs not used:
Ravenhill, Toney Booked: