Cavendish will manage relations with councillors, the Department for Transport and passenger groups.
Account director Gareth Morgan said: 'Unlike the big five transport groups – First Group, Arriva, GoAhead, National Express and Stagecoach – Transdev does not have to report to shareholders. We will represent its independent voice.'
Transdev also runs bus services in London and Bournemouth.
Previous incumbent agency The Waterfront Partnership worked with Transdev for over 18 months.