The England star scored a try and booted 16 points to send star-studded Munster crashing to defeat.
The Premiership side had to hang on at the death as the twice winners launched a ferocious onslaught on the home line.
But the Challenge Cup holders held on to complete a memorable start to their Heineken Cup campaign in a replay of the 2000 final won by the English club.
Fly-half Geraghty, watched by England chief Martin Johnson, said: "It was a special night for the boys. We've come through a tough game - I think they've been feeding the Irish boys raw meat all week.
"The game swung both ways, it seemed like every 20 minutes. But we backed ourselves and we've come a long way in the last couple of seasons."
Geraghty turned the tie Northampton's way with an opportunist score on the stroke of half-time.
He took a quick tap penalty when the Irish side expected him to kick for goal.
And it was Geraghty's 75th-minute penalty which finally killed off Munster's stirring fightback, led by Ronan O'Gara's boot and an opportunist try from Tomas O'Leary.
O'Gara was off target with an early penalty and it was Geraghty who edged Saints ahead with a ninth-minute penalty.
The ex-London Irish ace then set up the first try with a neat chip ahead fumbled by Munster full-back Paul Warwick.
The ball bounced kindly for winger Chris Ashton, who dotted down.
Warwick atoned for his mistake when he opened his side's account with a 50-metre drop goal.
And the Irish continued the fightback with a superb try after they had counter-attacked from deep in their own half.
Keith Earls spotted the home side were over-committed in attack. He kicked ahead and flanker David Wallace went over for a try that O'Gara failed to convert.
Geraghty and O'Gara slotted two penalties apiece to leave the game balanced at 14-14.
Another Geraghty penalty appeared likely when Munster were penalised for handling in a ruck, but the Irish province blundered by turning their backs on him.
The quick-thinking Geraghty tapped the penalty and scythed through for a try which he also converted. Munster knew they had gifted Geraghty that score but there was no doubting the quality of the try Northampton's fly-half set up for Ashton five minutes into the second half.
He made a superb break and passed inside to give the winger an easy run-in for his second of the night.
Geraghty took his points tally to 18 by adding the conversion but was off target with a 53rd-minute penalty.
O'Gara kept chipping away at Northampton's lead to set up a nailbiting finish.
O'Leary snaked over from a short-side break from a scrum close to the 22 and O'Gara reduced the lead to a point with a well-struck conversion.
But Geraghty had the final word, calmly slotting the winning penalty after Munster were penalised at a scrum.
Even then, Saints had to withstand a late forwards barrage before claiming a famous win.
SCORERS - Northampton: tries Ashton 2, Geraghty; cons Geraghty 2; pens Geraghty 4. Munster: tries Wallace, O'Leary; con O'Gara; pens O'Gara 4; drop goal Warwick.
This article has 1 comment
Who can blame him and he is the one that opted in all of his business ventures to be in the two riskiest types of business: launching a magazine and opening a restaurant. ,
By Coder47. Posted October 22 2009 at 12:06 PM.