Liberal Democrat Dean Sabri, who represented the ward of Hampden Park on the council, resigned due to new work commitments.
He was elected in 2015 and became the youngest councillor on the authority at the age of 21.
He served on the council’s decision making cabinet committee, deputy chairman of the town’s planning committee and more recently chaired the audit and governance committee.
He was also a leading member of former Eastbourne MP Stephen Lloyd’s campaign to get re-elected.
Lib Dem leader David Tutt said, “My thanks go to my friend and colleague Dean Sabri who resigned from the council this week.
Mr Tutt said, “In addition to his work on committees, he has helped very many residents who have brought their problems to him. His calm and considered approach to all that he does will be greatly missed.
“Many people believe that being a councillor is a full-time paid employment, not realising that in fact councillors receive a basic allowance of only £2,808 per year and need other work to survive.
“Dean now works for an organisation which demands political neutrality, hence the reason for his resignation. He takes with him thanks for his contribution to our local community and best wishes for his future career.”
It is likely that due to the Covid-19 pandemic, a by-election to replace Mr Sabri might not take place and the vacancy could remain open until the election of councillors in May this year.
Elections are planned for Thursday May 6 but this date may be brought forward or deferred by the government depending upon the circumstances.