Tom Sharp's 112 not out helped Cornwall defeat Hertfordshire by four wickets and make the final of the Minor Counties Knockout Trophy for the first time in their history.
George Scott hit 75 not out and put on 69 for the ninth wicket with Will Jones (20) as Hertfordshire made 269-8 from their 50 overs.
In reply, Cornwall slumped to 16-3.
But Sharp and Gavin Edwards (65 no) shared a seventh-wicket stand of 114 to seal victory.
Cornwall will play Northumberland at Wormsley in the final on Wednesday, 26 August.
Cornwall have never won the Minor Counties one-day tournament and have only won the three-day title once in their history, in 2012.