The hosts took a third-minute lead as Massadio Haidara's cross was stabbed home by Thauvin on his full debut, and things got worse for Northampton five minutes later.
The Magpies were awarded a free kick on the left-hand side, and Thauvin's cross was glanced in by midfielder Siem de Jong.
The Cobblers, buoyed by their goal, went close to equalising as Richards beat Mike Williamson to a cross and glanced it inches past the post.
Janmaat got his name on the scoresheet after the break as Thauvin slipped him in on goal and he slotted the ball through the keeper's legs from a tight angle.
The hosts wrapped up the win a few minutes later as centre-back Williamson nodded in Thauvin's corner to make it 4-1.