Mannix, a former All Black outside-half, scored a try and seven penalties for a 26-point tally, with a try by his fellow New Zealander Chris Yates completing Sale's account. The win lifts Sale up to fifth place in the First Division table and closer to their ambition of a European Cup place next season.
The Saints, still smarting from their Pilkington Cup 22-9 defeat by Sale last month, went ahead in the second minute with the first of Gregor Townsend's five penalty goals. Townsend and Mannix exchanged penalties in the first 20 minutes to keep the score level at 6-6 before Mannix eased Sale into a 15-6 lead with three more kicks.
Solid tackling from both sides restricted the attacking play in the first half, but the Saints went closest after a fine break by Jonathan Bell was stopped just short of the line by Jos Baxendell. Mannix went off on the half-hour for treatment to his nose but was back again after the interval.
Saints cut the lead to three points after the restart before Mannix charged down a kick by Townsend and raced 40 yards to touch down. Six minutes later Baxendell and Mannix linked up to put Yates over for a try, and with the last kick of the game Mannix emphasised Sale's superiority with his seventh penalty goal.
Mannix only revealed after the final whistle that he had played the second half with a broken nose. "I broke it when Phil Winstanley swung round and hit me with his head. It should be all right in about a week."
Mannix added: "It was a tough game and it was disappointing initially we could score only penalties but they are there to be taken. Defences are getting harder to open up and it was a lucky bounce that led to my try but it was great to score."
Sale: Tries Mannix, Yates; Penalties Mannix 7. Northampton: Penalties Townsend 5.
Sale: J Mallinder (capt); D Rees, J Baxendall, C Yates, T Beim; S Mannix, D Morris; P Winstanley, S Diamond, A Smith, D Baldwin, J Fowler, D Erskine, D O'Grady, J Mitchell.
Northampton: I Hunter; C Moir, M Allen, J Bell, H Thorneycroft; G Townsend, M Dawson; M Hynes, A Clarke, M Stewart, J Chandler, J Phillips, S Tatupu, J Cassell, T Rodber (capt).
Referee: Ed Morrison (Bristol).
n The Scotland flanker Ian Smith expects to be fit to face France in next Saturday's Five Nations' Championship decider in Paris despite suffering a cut eye playing for his new club Moseley on Saturday.
Smith came off a minute before the end of his debut for Moseley against Rugby Lions with a gash above his left eye.
The former Gloucester loose forward had seven stitches inserted in the wound but said: "It's nothing too serious. I have had it stitched and I should be all right for Paris."Reuse content