Northampton (14) 31 Tries: Dowson, Mujati, Diggin, Best Cons: Geraghty Pens: Geraghty 3 Saracens (12) 20 Tries: Melck Pen: Hougaard 4, Jackson
Mujati breaks through Ernst Joubert's tackle to score
Northampton punished Saracens' indiscipline with two second-half tries to set up an LV= Cup final meeting against Gloucester.
Phil Dowson's try made the difference in the first half, with Saints going in just two points ahead.
But with Saracens prop Richard Skuse in the sin-bin, Northampton ran through the depleted defence to set up tries for Brian Mujati and Paul Diggin.
Neil Best barged over to stretch the lead before Justin Melck's consolation.
Despite three successive losses coming into the match Saracens made the brighter start with an ambitious drop-goal attempt from Derick Hougaard inside the first minute.
It floated wide but the South African made amends soon after with the opening points from the tee.
Saints responded by driving downfield with big carries from Roger Wilson and prop Soane Tonga'uiha producing quick ball for captain Dowson to crash over from a few yards out.
Hougaard's decision to take on another drop-goal attempt from the halfway drew the wrath of the Franklin's Gardens crowd, though he redeemed himself in part with two successful penalties.
The two fly-halves traded penalties as Saracens emerged from the sin-binning of number eight Ernst Joubert with their lead thinned to 12-11.
But after missing one attempt at goal, Shane Geraghty took his side into the break ahead with a penalty after Skuse had been yellow carded for failing to roll away.
With Saracens still down to 14 men at the restart, Northampton took greater advantage of their numerical superiority.
First Lee Dickson's weaving break allowed Bruce Reihana to find prop Mujati lurking in space on the wing with a long looping pass.
Diggins' try quickly followed as Best switched play to Geraghty who flipped the ball out the tackle for Chris Ashton to race clear and put his fellow winger in.
Best then forced his way over from a huge overlap on the right flank, leaving Melck's training ground score in vain as Saracens struggled to make use of their own man advantage in the last 10 minutes.
Northampton will face Gloucester for the season's first silverware at Worcester's Sixways stadium on 21 March.
Northampton director of rugby Jim Mallinder:
"In the second half, we were playing down the slope and we knew if we could put some phases together we could open them up.
"We still like to play and we moved the ball well when we had the opportunity. I'm delighted with the win.
"The final is going to be a hard game. Gloucester have improved massively since the start of the season and they will go in fancying themselves to beat us."
Saracens director of rugby Brendan Venter on Richard Skuse's yellow card: "That was hugely significant. We were leading by a point and 10 minutes later, they have scored 13 points.
"It killed the game for us.
"The most positive thing was that the best player on the pitch, Northampton prop Soane Tonga'uiha, will be playing for us next season."
Northampton: Reihana, Ashton, Clarke, Downey, Diggin; Geraghty, Dickson; Tonga'uiha, Sharman, Mujati, Fernandez Lobbe, Kruger, Dowson (capt), Best, Wilson. Replacements: Myler for Downey, Dickens for Dickson, Dreyer for Tonga'uiha, Long for Sharman, Cannon for Fernandez Lobbe, Easter for Dowson.
Not Used: Vickers, Ansbro.
Saracens: Goode, Penney, Ratuvou, Barritt, Wyles, Hougaard, de Kock, Lealamanua, Brits, Skuse, Smith, Botha, Burger, Saull, Joubert (capt). Replacements: Haughton for Wyles (42), Jackson for Hougaard (50), Marshall for de Kock (50), Mercey for Lealamanua (11), Ryder for Smith (54), Melck for Saull (54), Brown for Mercey (63).
Not Used: Reynecke.
Referee: R Poite (FRFU).