A first-half blitz from Jimmy Floyd Hasselbaink's side saw them go into the break with a 3-0 advantage and three points as good as in the bag.
Lawson D'Ath pulled a goal back for Northampton on 53 minutes but Chris Wilder's side, after a positive campaign, now know that they will be in League Two again next season.
Wilder spoke of Burton as a benchmark for the Cobblers before the game and they showed why they will be playing at a higher level in 2015/16.
Town end the season at Sixfields against Wycombe Wanderers next Saturday.
Reaction: Chris Wilder spoke to the official NTFC website afterwards and said: "We will need to show what we did in the second half today on a much more consistent basis if we are going to get out of this league.”