The Sharks ran in three second-half tries from Mark Easter, Dwayne Peel and David Seymour to continue their encouraging start to the campaign.
The visitors had forwards Calum Clark and Phil Dowson sin-binned midway through the second half and Sale took advantage by overpowering Saints.
All Northampton's points came from the boot of fly-half Stephen Myler, who kicked seven penalties, but it was not enough to prevent a third straight defeat.
Sale trailed 12-6 at half-time but upped the tempo after break and took full advantage of Clark and Dowson's yellow cards.
Diamond said: "What we've done is take all the excuses out and given the players the best possible facilities and knowledge.
"At half-time the lads had not performed to the level that we know we can perform.
"It wasn't a rollicking session at half-time. We just said that if we get the ball out wide then we could score tries and we did that, courtesy of them having 13 men.
"You watch how Northampton play and attacking them out wide behind their big centres is the best way to get at them."
Diamond presided over a major overhaul at Edgeley Park during the summer in a bid to transform Sale into a serious force again.
The 2006 Premiership champions have fought relegation in recent seasons and Diamond, who saw Peel sin-binned in the first half for kicking the ball away, said: "We've got a lot of new players here.
"The lads who were fortunate to be offered a new contract and the other lads who came in a have a desire to achieve and succeed.
"That's down to the players and the team spirit is generated on the training field, not in a bar or a bowling alley."
Saints director of rugby Jim Mallinder felt Clark's yellow card for obstruction was harsh but had no complaints with Dowson being sin-binned for kicking the ball away.
However, he felt referee Tim Wigglesworth had done his team no favours.
Mallinder said: "You don't want to blame referees and we talked about being squeaky clean, but quite obviously in the first 10 minutes the referee was very keen in terms of the breakdown, of rolling away in the tackle and being onside.
"We talked about it but we were still getting penalised.
"The second yellow card was tough. It's tough enough being one man down, even tougher when you are two men down."
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