The domination of the Saracens forwards proved decisive as the Saints pack came under increasing pressure and were punished with a string of penalties.
Glen Jackson, the Saracens fly-half, produced a faultless goal-kicking display to make Northampton pay for their indiscretions, landing seven penalties and converting a try from full-back Dan Scarbrough for a personal haul of 23 points.
Northampton full-back Bruce Reihana also took all his kicking chances, with two conversions and a penalty. Saracens looked to be heading for defeat after winning their last four matches in the Guinness Premiership and Heineken Cup, their defence ripped apart twice in a four-minute early spell.
Reihana and centre Jon Clarke began a 70-metre move which ended with Scottish international winger Sean Lamont sending Robinson over after only six minutes. Four minutes later Clarke went through two tackles on a run from the halfway line before handing Robinson a simple second try and Northampton were 14-0 ahead, with Reihana converting both.
While the visitors were more dangerous in the backs, Saracens had a definite edge in the forwards and this ultimately proved crucial. Their extra power up front resulted in a 20th minute yellow card for Northampton second row man David Gerard.
Northampton defended desperately on their own line as their depleted forwards took a pounding but their defence was found wanting as Saracens scored their only try of the match.
Right-wing Richard Haughton cruised past five hapless would-be tacklers before being hauled down just short of the line but he left the ball for Scarbrough to follow up and score. A penalty from Reihana put Northampton 17-7 ahead but Jackson replied with four penalties for Saracens to put them in front for the first time at 19-17 in the 49th minute.
He struck three more to increase the Saracens lead to 28-17 before Northampton’s former All Blacks fly-half Carlos Spencer scored Northampton’s third try in the right-hand corner late into injury time.
But despite running in three tries to the home side’s one, Northampton finished a well-beaten side.