The Cobblers’ travel sickness showed no signs of easing at Rotherham as the home side romped to a comfortable victory.
Town had no answers to a team that - along with Manchester United - are the only club in the Football League not to have drawn a home game this season.
There was no danger of such an outcome in this match, with Northampton making the Millers look like the giants of Old Trafford in the form of a 3-1 scoreline improved only cosmetically by a late Clarke Carlisle header.
Rotherham’s inconsistencies were well documented in the build-up to the game but while they are undoubtedly a talented squad they went into the game on the back of a three-game losing streak.
It started so well - Adebayo Akinfenwa showed some good feet on the edge of the Millers box before rattling off a shot to which Andy Warrington, diving low to his right, was the equal.
Kayode Odejayi did likewise at the other end after debutant Jordan Slew bustled his way - somewhat fortuitously - through a couple of challenges before sliding the pass across.
In what was a fairly open start to the game, Jake Robinson then cut inside off his wing and fired a right-footed shot that dipped past Warrington and against the post, with Mark Bradley pouncing on the rebound to deny the arriving Akinfenwa.
Things started to turn five minutes from half-time when Ben Harding was harshly adjudged to have fouled Bradley in the box - it appeared Harding got a toe to the ball but the referee had other ideas and pointed to the spot.
Daniel Nardiello, who had barely been in the game up to that point, made no mistake, sending Nicholls the wrong way.
Rotherham started the second half brightly, Tozer blocked at the feet of Odejayi and David Noble steering a shot into Nicholls’ hands after a midfield raid.
Town had their moments too, notably when Akinfenwa slipped a pass out to Chris Hackett, whose first shot was blocked by two covering players and his second by a combination of Warrington and a third Rotherham defender.
But their attacks were rarely sustained periods of pressure on the home side’s goal and the match was settled when the ball broke to Ben Pringle to fire a Rotherham second.
The ball squirmed to the central midfielder 20 yards from goal, he took a touch and then rattled a left-footed shot that deflected - but not by much - past Nicholls and into the corner.
The goalkeeper made a great save to deny Nardiello after he had wandered in behind a ball-watching Tozer but it only delayed the third goal, which came when Noble took the ball in his stride, controlled it on the bounce and despatched it with style.
Rotherham could have added to their tally and certainly finished the game as the stronger side, even if Carlisle did socre a consoling header with the game’s final touch.
The Cobblers’ promotion credentials, meanwhile, continue to be undermined by a travel sickness that often leaves everyone feeling as green as their glow-in-the-dark kit.
Rotherham: Warrington, Bradley, Arnason, Mullins, Ridehalgh, O’Connor, Pringle, Noble, Nardiello, Slew, Odejayi
Subs: Agard (for O’Connor 84), Kearns (for Noble 86), Cameron (for Nardiello 90+2)
Not used: Thompson, Revell, Morgan, Rowe
Cobblers: Nicholls, Tozer, Langmead, Carlisle, Widdowson, Hackett, Oyeleke, Harding, Guttridge, Robinson, Akinfenwa
Subs: O’Donovan (for Oyeleke 46), Platt (for Robinson 71)
Not used: Johnson, Demontagnac, Snedker, Moult, Hornby
Away fans: 956