Aidan Carter's half century and a fine bowling display saw Cornwall edge out Yorkshire by 10 runs (DLS) in a rain affected encounter at Stormont.
Jay Smith (1-32) gave Yorkshire an early breakthrough after the first of three rain delays on a stop-start day in Belfast.
Spinner Werner Beukes then claimed four wickets in quick succession to leave Cornwall reeling at 54 for 5.
Cornwall were rescued by a 6th wicket stand of 88 between Aidan Carter (51) and Jacob Inch (39), both of whom struck five boundaries.
Rob Heyes (2-32) and Marcus Hayes (3-15) cleaned up the tail as Cornwall sought quick runs as the overs ran out.
A final total of 167 in 47 overs looked about par in testing conditions and so it proved in a tense second half.
Yorkshire were set 128 in 35 overs, and despite a fine 20 from Reece Slater were always just behind the required rate.
Wickets for Connor Cooke, Ben French, Adam Snowden and Freddie Stevenson had Yorkshire 72 for 5 in the 25th over when the players were forced from the field for the final time just after 6 o'clock, Cornwall declared winners, 10 runs ahead on DLS.