He had trials for local professional football club Middlesbrough, and although he was not able to join the club as a professional due to arthritis, he still supports them. He left school with three A-levels and went on to study law at the universities of Sussex and Leicester. There, Mortimer became involved in political causes and the punk movement, starting a band called Dog Dirt. After leaving university with an LLM in Welfare Law, Mortimer moved to London and became a solicitor for Southwark Council. He then moved to a private practice in Peckham, where his work with Public Health Act cases regarding cockroach infestation of council properties led to a local paper dubbing Mortimer "The Cockroach King".