Cllr Liam Maxwell has resigned from the Royal Borough to take up a new, politically restricted assignment with the Cabinet Office in London.
The borough's Cabinet Member for Policy and Performance, Cllr Maxwell has represented the Eton and Castle ward in Windsor since 2007.
Ian Trenholm, Chief Executive, said: "Cllr Maxwell's new assignment, as is common with both senior civil service and council roles, is 'politically restricted', which means he cannot serve as a councillor at the same time as carrying it out. As a result Cllr Maxwell has tendered his resignation from the Royal Borough. We would like to thank him for the work he has done to represent local people and wish him well in his new assignment.
"At this stage we have no information about a by-election to fill the Eton and Castle ward vacancy."
The date for the by-election will be set once a request to fill the vacancy is received by the returning officer. The request must be from two registered electors in the borough. The by-election would then be held within 35 working days.