The central defender will not be returning to Woodside Road next season, instead joining former Worthing team-mate Dajon Golding at National South side Cheshunt.
The 23-year-old joined the Rebels from Leatherhead last summer and made his debut away to Folkestone Invicta on the first day of last season’s title winning Isthmian League campaign.
Seager made a total of 45 appearances for the Rebels scoring two important goals from defence, the first in an away Sussex Senior Cup quarter-final victory against Bognor at Nyewood Lane in January, the other away to his new club Cheshunt during the title run-in. Both came in 2-0 wins to the Mackerel Men.
His final appearance for the Reds was a start in the 4-2 Sussex Senior Challenge Cup final defeat to Brighton & Hove Albion at the start of May.
Worthing FC said: “The club would like to thank Will and wish him well for the future.”
Seager and Golding are the only two departures from last season’s squad, while the club have already announced the likes of Callum Kealy, Ollie Pearce, Aarran Racine and Dean Cox are staying with them.
In addition they have signed experienced trio Jake Robinson, Kane Wills and Adam El-Abd and exciting forner Carshalton forward Lewis White.
Worthing’s first friendly is less than a fortnight away – they host Isthmian south east side Three Bridges at 7.45pm on Tuesday, July 5. Pre-season training starts three days before that.