Since 1988, Professor Slevin was a consultant clinical oncologist, before retiring in 2024. He specialised in head and neck cancer integrating clinical practice with clinical research and produced over 150 papers. He was Chair of the Radiotherapy CRG (NHS advisory group), the first Chair of the NCRI head and neck research group and was Associate Medical Director, non-surgical oncology at The Christie.
Prof Slevin was also the SRO (Senior Responsible Officer) for the Manchester proton build, the first proton centre in the UK.