Havant lost to Lymington on the final day of the Southern Electric Premier League division one season.
And with it they slipped to third spot in the final table.
Opening batsman Chris Morgan (34) anchored the Havant innings early on.
Chris Stone (30) and Andrew Gorvin (22 not out) helped the home side reach 158 all out.
But despite losing a couple of early wickets, the New Forest visitors cruised home for a seven-wicket success.
Ventnor lost by 18 runs at Totton & Eling but secured their top-flight status.
A third-wicket partnership of 135 helped the home side set a target of 265 with Charlie Freeston (four for 36) and Adam Hose (three for 53) the pick of the Ventnor bowlers.
Jacob George and Ollie Mills put on a 58-run for the first wicket but the islanders came up short, despite contributions by Jamie Miller (64) and Hugh Calloway (50).