The Saints had lost all five of their previous Premiership semi-finals, but they roared into a 17-0 half-time lead with quick-fire tries from Brian Mujati and Jamie Elliot.
Owen Farrell kicked two penalties to keep alive any faint hopes of a Saracens comeback - but they were quashed when GJ Van Velze scored Northampton's third just before the hour.
Saracens did engineer a try from Duncan Taylor when Elliot was in the sin-bin but Northampton's grip on the game was vice-like and Stephen Myler's second penalty, on top of his three conversions, wrapped up a famous victory.
Northampton had finished 12 points behind table-topping Saracens in the regular season. They had lacked consistency at times but produced one of their performances of the campaign.
Perhaps only their victory away at Ulster in the group stages of the Heineken Cup could match it.
Northampton's shuddering physicality dominated the first half and it was only due to two missed penalty kicks from Myler that the match remained scoreless for 20 minutes.
Saracens were out of sorts. Charlie Hodgson did release Alex Goode with a glorious pass but Northampton closed the space down and Farrell's grubber kick was too heavy and skipped dead.
Saracens lacked composure in the face of a high-intensity performance from Northampton that was personified by Samu Manoa, the American international who was acting as a one-man destroyer from number eight.
Not many defenders stop Mako Vunipola in his tracks but Manoa did it twice and he was ably supported by Saints captain Dylan Hartley, who led from the front.
Northampton's power engineered the breakthrough, with a lineout drive try for Mujati, and the Saints raced into a 17-0 lead by the interval.
Calum Clarke claimed the lineout and Northampton drove for the line, with Mujati stretching out an arm to touch down for a try that was awarded by television match official Graham Hughes.
Myler converted from wide on the right and, just two minutes later, he was successful from the left wing after Northampton scored a scintillating second try through Elliot.
Manoa laid the foundation with a crunching tackle on Chris Ashton and Northampton launched a counter-attack from the turnover, with Hartley and Christian Day combining to release Elliot with the scoring pass.
Farrell, who had been moved to fly-half after Hodgson went off injured, was penalised for a late tackle on Myler, who dusted himself off to extend Northampton's lead.
Saracens ended an ignominious 40 minutes with a missed penalty from Farrell, leaving them with a mountain to climb in the second half.
Two years ago, Northampton allowed a 22-6 half-time lead to slip against Leinster in the Heineken Cup final and Saracens began the second period with greater urgency.
David Strettle sliced through the Northampton line but he was halted by James Wilson and then denied by some quick-thinking from Ben Foden, who slid under the Saracens wing as he twisted for the line.
Saracens, despite the presence of Lions props Matt Stevens and Vunipola, saw their subsequent scrum destroyed but they kept the pressure on and the door ajar with successive penalties from Farrell.
But with an hour gone, that door was slammed firmly shut by Van Velze, who galloped over for Northampton's third try, having come on as a replacement for Courtney Lawes.
Northampton secured lineout ball on the Saracens 22, Hartley carried the ball forward and then recycled quickly for Lee Dickson, who spotted an opening down the short side and released Van Velze.
Northampton were reduced to 14 men when Elliot was sin-binned for taking Farrell out in the air and Saracens took advantage, eventually working space for Taylor to score the try.
Saracens had their tails up but Northampton defended valiantly, forcing handling errors and they saw out the rest of the sin-bin period without further damage.
And when Phil Dowson stripped Strettle of possession, Northampton piled forward and won a penalty which Myler kicked as the Saints marched on to Twickenham.
Leicester sealed their place in a ninth consecutive final yesterday with a 33-16 victory over 2012 champions Harlequins.
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- Owen Farrell