The Millers made it three stalemates on the bounce but have now not won in their last 10 attempts in all competitions.
Gareth Evans gave Rotherham the perfect start with a goal in the second minute. Keeper Sam Walker made a hash of a clearance and Danny Schofield set up Evans, who cut inside to open the scoring.
Northampton got back on level terms in fortuitous circumstances in the 22nd minute. Lewis Young drove in a low cross to the near post where keeper Conrad Logan failed to deal with it and the ball squirmed through his hands and into the net.
Northampton then enjoyed their best spell, with Kelvin Langmead twice off-target before the break. Northampton went in search of the elusive winner in the second half, with Adebayo Akinfenwa seeing a powerful header superbly saved on the line by Logan, while Ashley Westwood headed wide late on at the near post from a Michael Jacobs' corner.
In between, substitute Luke Foster missed a great chance to win it for the Millers when he raced onto Schofield's pass but shot wide from a good position and Danny Harrison's header was cleared off the line.
Northampton almost snatched victory in stoppage time when on-loan Derek Niven saw his shot deflected wide for a corner.
Source: DSG