Colin Calderwood enjoyed a happy return to Northampton as Forest maintained their 100% start to the season courtesy of Chris Doig's own goal.
Jack Lester spurned an early chance to give Forest the lead after Doig upended Grant Holt inside the box, but he blazed the penlaty over the bar.
Doig then deflected Holt's shot past Paul Smith before Scott McGleish was sent off for Northampton.
Sammy Clingan almost doubled the lead late on but his shot hit the post.