South African left-arm seamer Pietersen took seven wickets for 10 runs as the hosts were bowled out for 56 in under 25 overs, extras top-scoring with 24 and no batsman reaching double figures.
Northamptonshire then cruised to an eight-wicket win.
Elsewhere England captain Michael Vaughan's first match of the season saw him spend some time in the field with Yorkshire over Ireland in Belfast.
But it was his England team-mate Matthew Hoggard who made the most of a rain-affected day by taking two wickets as Ireland reached 33 for three in the 12.3 overs possible.
The match, as with all of Tuesday's weather-affected games, is due to be played to a finish on Wednesday.
Andrew Cook (162 not out) set the highest innings by a Lancashire batsman in a One-day game with his magnificent knock against Buckinghamshire at Wormsley.
Cook, making his tournament debut, was joined at the crease by Glen Chappel who smashed the fastest half-century in the competition's history, hitting 55 off 16 balls.
The visitors reached 370 for four after their 50 overs, then James Anderson struck twice to reduce the minor County to 39 for two before rain intervened.
Holland have an outside chance of victory against Warwickshire on Wednesday, when they will resume needing 137 more runs from 23 overs.
The Bears posted a modest 237 for five from their 50 overs in Rotterdam, the innings held together by skipper Nick Knight (108).
Hopes of a home victory rest with 22-year-old Daan van Bunge, who is 37 not out overnight with his side 101 for four when play was stopped for the day.
No play was possible at Exmouth so Devon will face Essex on Wednesday, and fellow minor county Northumberland will also have a delayed start to their tie against Middlesex at Jesmond.