Second-half goals from Daryl Dike, Karlan Grant and Adam Reach saw Steve Bruce's side stroll to victory in a friendly at Sixfields.
The Baggies created four decent chances in the first half with Jed Wallace and Dike twice going close.
But they improved significantly after the interval and thoroughly deserved to win by such a comfortable margin.
The League Two Cobblers created a couple of chances for strikers Mitch Pinnock and Danny Hylton.
But in truth, Jon Brady's side were comfortably brushed aside by the Baggies in what was their third pre-season outing of the summer.
Bruce opted to line up his side in a 4-3-3 formation at Sixfields.
The system saw Jed Wallace start on the right wing with Callum Robinson on the left and Daryl Dike the central striker.
In the middle of the park, Jake Livermore was the deepest of three midfielders with Jayson Molumby and Adam Reach either side.
Northampton, who included former Walsall man Jon Guthrie at centre-back, lined up in a 4-4-2 system.
It was the Baggies who flew out the blocks with Bruce's men creating three good chances to open the scoring inside the first 15 minutes.
Two of them fell to new boy Wallace with the winger first capitalising on some loose defending before entering the box and firing wide.
Just moments later Wallace was in once again when he was slipped through by Dike.
But while this time he got his shot on target, his effort was beaten away by goalkeeper Jonny Maxted.
Albion were consistently knocking on the door and they threatened again when Molumby cushioned the ball down to Dike in the box.
From 12 yards, though, American blasted the ball over the bar when he should have done better.
In their previous outing at Stevenage, Albion had been good for half an hour before allowing the Boro to grow in stature.
And the same happened here with Brady's men getting more confident with each passing minute.
And they came a whisker away from opening the scoring when Akin Odimayo delivered an excellent ball to Louis Appere who then laid it off to Mitch Pinnock in the box.
The former Arsenal man then curled a strike that was heading straight into the top corner.
Darnell Furlong, though, did outstandingly well to race back and somehow head the ball over his own crossbar.
From that point on the half proved more cagey with little goal mouth action.
Albion did, though, have one more glorious chance before the break when keeper Maxted got in a muddle and dropped the ball at the feet of Dike.
The striker looked set to have a tap-in. But Maxted atoned for his error by brilliantly blocking the ball at the American's feet.
It was in the 51st minute that Albion did break the deadlock.
Wallace did well to charge down the right and find Dike with a cross.
And he then swiveled well before firing a low drive through the legs of Maxted and in.
Bruce made a double change shortly after with Grady Diangana and Matt Phillips replace Molumby and Dike.
It was a move that saw Albion switch to a 4-2-3-1 system with Karlan Grant, who had earlier replaced Robinson, up front.
Diangana operated on the left with Phillips in the number 10 role allowing the lively Wallace to continue on the right.
As is often the case with friendlies, replacingthe substitutions continued to come with Alex Mowatt and John Swift replacing Conor Townsend and Livermore.
And Albion came close to doubling their lead just minutes later when a flowing move ended with Mowatt seeing a shot beaten away by Maxted.
The Baggies were now playing a flowing system that had Diangana, Swift, Wallace and Phillips all operating behind Grant.
And they doubled their lead when Grant found the bottom corner after being played in by an exquisite Diangana pass.
Northampton almost got a consolation with 10 minutes to go when the ball broke from Danny Hylton in the box.
But he fired over when he should have done better.
But with just a few minutes to go, Albion went on to grab a third.
Diangana hit a powerful drive in the box that smashed against the post.
But the ball then fell for Reach whose effort squirmed through and sea of bodies and over the line.
Albion (4-3-3): Button (Palmer 46), Furlong, Ajayi, O'Shea, Townsend (Mowatt 65), Livermore (Swift 65), Molumby (Phillips 56), Wallace (Gardner-Hickman 76), Robinson (Grant 12), Reach, Dike (Diangana 56).
Northampton (4-4-2): Maxted, Sowerby (Abimbola 73), Guthrie (Dyche 81), Sherring, Hoskins (Connor 76), Fox (bowie 50), Appere (Hylton 46), Pinnock (Trialist 73), Koiki (Haynes), McWilliams (Cross 74), Odimayo.