The Steelbacks, who had won the toss, were soon reeling as Joe Leach took a hat-trick, and the away side limped to 19 for six before Josh Cobb and Rory Kleinveldt steadied the ship.
David Ripley’s men eventually made 126, with Worcestershire seemingly set for a comfortable win.
But the bowlers did the business, with Muhammad Azharullah’s four wickets key in a stunning 21-run success on a memorable afternoon.
The victory is the Steelbacks’ second in the Royal London One-Day Cup group stages and keeps alive their hopes of qualifying for the quarter-finals ahead of tomorrow’s clash with Somerset at the County Ground.
But it was a win that seemed unthinkable early on as Leach became the first bowler to take a hat-trick with the first three balls of an innings in county cricket history.
Former Northants man Chaminda Vaas was the only other bowler to do it in the professional game worldwide, for Sri Lanka against Bangladesh in the 2003 World Cup.
Northants captain Alex Wakely, who was returning to the side after recovering from a finger injury, had opted to bat, but Richard Levi got a thin edge pushing forward from Leach’s opening deliver.
Rob Keogh followed in identical fashion before Ben Duckett fended the hat-trick ball to be caught in the gully.
Not to be outdone, Jack Shantry responded with three wickets at the other end as the Steelbacks found themselves six wickets down inside eight overs, Wakely fourth to go cutting straight to point on eight.
David Willey tried to take the attack to Shantry, but only succeeded in getting a leading edge to mid-off, before Steven Crook flashed at his fourth ball and was caught behind later the same over.
It could have been worse but Kleinveldt was dropped at third slip which would have made it 27 for seven, and although it seemed immaterial at the time it would prove a crucial moment as together with Cobb the South African added 50 for the seventh wicket.
Kleinveldt clubbed Shantry for three consecutive boundaries as he took the aggressive approach, hitting seven fours in total before he miscued a pull off Ed Barnard.
Cobb had survived the early carnage, dropping anchor in scoring 20 from the first 24 overs before pulling Barnard for six, however Leach soon returned and although Graeme White was dropped at third slip, Cobb edged the next ball into the cordon.
White took the score into three figures, but Shantry removed him with his fourth wicket via a top edge and although Stone hit a four and six, Azharullah swept Saeed Ajmal straight to fine leg to end the innings.
With Northants bowled out in 35 overs, Worcestershire were faced with just under an hour to bat before the lunch interval, and after reaching 16 without loss, they lost three wickets for three runs.
Tom Kohler-Cadmore guided Stone to slip, before Tom Fell edged Kleinveldt to Duckett and Azharullah trapped Joe Clarke LBW with his second delivery.
Brett D’Oliveira was caught at slip off Azharullah to leave the hosts 29 for four at lunch.
Daryl Mitchell and Ross Whiteley added a further 12 after the interval before the latter aimed a big swipe off Azharullah and edged to Duckett.
And in Azharullah’s next over, Mitchell gloved a catch down the leg side for 18 to leave the hosts 48 for six, still 79 runs from victory.
Hat-trick hero Leach hit five boundaries on his way to 21 from 10 balls but got a top edge off Stone to be caught at third man.
And two overs later, Barnard misjudged a single that was never on to Crook at mid-on, Duckett removing the bails to beat the desperate dive.
Willey had to wait until the 28th over to come into the attack, but he only needed four balls to make an impact as Shantry was caught by Levi, his third catch in the slips, without score, and leave the host 38 adrift as last man Ajmal joined Ben Cox.
Cox had made his way to 21, and hit three more boundaries to take the score past 100, but on 36, the highest score of the day, he tried one shot too many and his top edged cut was well caught by Stone running backwards at cover, sparking wild celebrations among the Northants fielders.
Worcestershire Rapids: Mitchell (c), Kohler-Cadmore, Fell, Clarke, D’Oliveira, Whitely, Cox (w), Leach, Barnard, Shantry, Ajmal.
Northants Steelbacks: Levi, Cobb, Wakely (c), Keogh, Duckett (w), Willey, Crook, Kleinveldt, White, Stone, Azharullah.