Martyn was a Police Officer in both the Metropolitan Police and Sussex Police from 1979 to 2009. He retired as a Detective Chief Inspector, having been a Senior Investigating Officer or Deputy Senior Investigating Officer in various murder enquiries, including that of a the abduction and murder of Sarah Payne. During his service he gained a Diploma in Policing at Sussex University and a Masters Degree in Criminology at Portsmouth University as well as a PGCE.

He also became a qualified Assessor and Verifier.

On retirement, he was then elected as Dorset's Police and Crime Commissioner from 2011 to 2021, after which, he stood down and joined Bournemouth University as a Senior Lecturer.

Martyn now works in the Health and Social Sciences Faculty, and is joint Programme Lead of the BA in Criminology and Law Programme, and Unit lead of the Human Trafficking and Criminality Unit.