Spencer did not enjoy a happy Rodney Parade debut
Dragons: (16) 26
Tries: Wyatt, Fussell
Cons: Sweeney (2)
Pens: Sweeney (3)
DG: N Brew
Northampton: (7) 7
The Dragons ended Northampton's EDF Energy Cup hopes - but Leicester's bonus-point win means that the Welsh side cannot make the semi-final either.
Much pre-match talk was of Carlos Spencer, but he was sin-binned early on, and outshone by opposite number Ceri Sweeney who kicked 13 points.
Sweeney sent Gareth Wyatt in for an early try, Mark Robinson responding with a five-pointer for the Saints.
Colin Charvis sent Richard Fussell in for the decisive try after the break.
Saints outside-half Spencer was playing only his second game in Wales, and the Kiwi superstar soon made a mark - with a spike tackle on Gareth Cooper that earned him 10 minutes in the sin bin.
Sweeney landed the resultant penalty, and - to the delight of the Rodney Parade crowd - the Dragons proceeded to take advantage of the extra man.
From good ball in the Northampton 22, Cooper fired the ball into midfield and Sweeney fed Gareth Wyatt who ran through the midfield untouched for the opening try.
It was imperative for us to get a home win
Man-of-the-match Colin Charvis
The visitors protested in vain that Nathan Brew's dummy run should have been called as crossing.
Sweeney added another penalty before the Saints rallied with an extended attack that ended with Mark Robinson dummying past Cooper for a try converted by Robbie Kydd.
With Spencer off with a groin strain, Sweeney notched a further penalty on the stroke of half-time to take the lead back out to 16-7, and the Dragons started the second period on fire.
The Gwent side battered at the visitors' defence for five minutes before inspirational man-of-the-match Charvis found a gap and fed Fussell beautifully.
Cooper made an encouraging first start after injury
The replacement wing was brought down yards short, but was not held in the tackle and was able to wriggle forward for a try under the posts, converted by Sweeney.
The Saints rallied, and their hopes rose when Welsh prop Rhys Thomas was sin-binned for using his hands to delay the ball at a ruck.
But they failed to make the numerical advantage count and saw Brew seal the victory with a snap drop goal.
Newport-Gwent Dragons coach Paul Turner on Colin Charvis:
"Colin was the best player on the pitch by a country mile.
"He has got the right leadership qualities, and he did a few things tonight that kept us in the game at certain stages.
"He is like most very good international players in that he doesn't make a lot of mistakes."
Northampton boss Paul Grayson:
"I thought the game was disappointing for both teams, especially when you consider both sides needed to score four tries to take the pool into the last week.
"The second half was just one stoppage after another, and we just didn't do the basics well enough.
"We were still in there swinging at the end - there just needs to be a little bit more guile."
Man-of-the-match Colin Charvis:
"Some people may think this competition is taken lightly, but it was imperative for us to get a win in front of our home crowd.
"Kevin [Morgan] led us well and I thought that the whole team went fantastically."
Dragons: Morgan, Wyatt, N. Brew, Smith, A. Brew, Sweeney, Cooper, Black, Daly, R. Thomas, Gough, Sidoli, Charvis, Ringer, Fitisemanu.
Replacements: Fussell for Wyatt (42), Warlow for Smith (80), Crawford for Daly (79), Charteris for Gough (64), Owen for Fitisemanu (53). Not Used: Harrison, Walker.
Northampton: Kydd, Lamont, Wyles, Quinlan, Diggin, Spencer, Robinson, Smith, Hartley, Barnard, D. Browne, Short, Lewitt, Fox, D. Browne.
Replacements: Laird for Kydd (78), Howard for Diggin (60), Vass for Spencer (40), Richmond for Hartley (78), Budgen for Barnard (65), Gerard for Short (66). Not Used: Easter.
Ref: Nigel Owen (WRFU).