The only goal arrived 11 seconds after the restart when a poor error of judgement by Jason Crowe allowed man-of-the-match Grant Holt to deliver a low cross, which Chris Doig could only hammer into his own net in attempting to clear.
Doig was involved in the game's first major incident when he conceded a third minute penalty for hauling back Holt, but Jack Lester made it three penalty misses from three this season for Forest, by clipping the spot-kick over the crossbar.
There was plenty for fans to talk about in this quick return for Calderwood against his old mate John Gorman, as Northampton's skipper Scott McGleish was shown the red card in the 61st minute for apparently elbowing Julian Bennett, although Gorman is adamant after watching a video of the incident that his club will appeal.
This was Forest's first ever League win over Northampton at the seventh attempt and it came 40 years after the sides last met. It also inflicted the Cobblers' second home defeat inside five days and Gorman knows with limited resources the challenge ahead is a significant one.
Gorman sprang a surprise for home fans by giving a league debut to 21-year old goalkeeper Mark Bunn in place of Lee Harper and the youngster could hardly have expected to face a penalty so quickly, although he was not called upon to save it.
Town had first-half chances and could have gone ahead in the 20th minute after a free-kick from Andy Holt rebounded into the path of McGleish, although the shot on goal eventually came from Ian Taylor, the veteran midfielder poking the ball into the sidenetting.
From another cross by Holt, McGleish towered above the Forest defence but couldn't direct his header down far enough.
At the other end rookie goalkeeper Bunn was out smartly to foil Holt just before the break, but it was Crowe's error straight after the re-start which proved decisive.