Wales comfortably chased down 404 in the final innings to beat Cornwall by five wickets at Usk in the Minor Counties Western Division.
They reached the target with 25 overs to spare, the highest successful chase ever achieved against Cornwall, having trailed by 156 on first innings.
In a game of records, it was also the first time 400 had been scored by both sides in any Cornwall match.
Having topped the table for some time, Cornwall finished fourth.
However, they could still take some silverware from the season, as they play Northumberland in the Minor Counties one-day final at Wormsley on August 26.