It was 1 - 1 after just 5 mins but Town went into the break 2 - 1 up and went on to win comfortably.
"It's a great win for us and the manner of the victory was very pleasing," said boss Stuart Gray.
"We were disappointed not to get the three points against Forest last weekend but we thoroughly deserved this victory although we could have and perhaps should have scored more goals.
"We didn't move the ball about as quickly as we should have done in the first half but in the second half we passed it round well and I was really pleased with our attacking play."
Northampton now face an away game at Doncaster on Boxing Day.