Dennis has been a Judge and an Associate Judge of the NSW District Court and a NSW Supreme Court Common Law Arbitrator and Mediator. Dennis has been a member of the Bar Council of NSW for many years.

He is a Captain in the Royal Australian Naval Reserve, a defence force Magistrate and a member of the Judge Advocates Panel. He was the commissioner who enquired into the Falcon Airlines and related matters. He has been on the final selection panels for the appointment of senior counsel in both NSW and the ACT. Dennis has been a long time member of the adjudicators' panel at the Australian Golf Club and a judicial officer and chairman and member of the IRB Appeal Panel for Rugby World Cup 2015.

Dennis has also been awarded the Reserve Forces decoration and an Australian Military Service medal. He was a prosecutor for the Commonwealth Crown Solicitor before entering private practice