Riddell began working in tourism in 1978, first assistant tourist officer with Scottish Borders Council, then as Chief Executive of Scottish Borders Tourist Board, joining VisitScotland in 2005 as Director of Partnerships, Strategy and Communications and latterly as Director of Industry and Destination Development. Riddell led and managed a team of over 300 staff that delivered services and advice to visitors, tourism businesses and a wide range of external stakeholders. Riddell’s team operated the network of i centres and delivered VisitScotland’s star rating quality assurance schemes. He was responsible for developing the organisation’s Gaelic Language Plan, Responsible Tourism Strategy and led VisitScotland’s input to the Scottish Agritourism strategy, supporting the formation of the Scottish Agritourism Board in May 2022. Since his retirement in September 2020 he has taken on a series of roles including chairing a group of voluntary Trustees responsible for a Grade A listed building in Eyemouth and a community liaison group linked to a major pump storage hydroelectric scheme north of Fort William.