Bentley received a Bachelor of Arts degree from the University of Western Ontario in London, Ontario, and obtained a law degree from the University of Toronto in 1979. Two years later, he obtained a Master's degree from Cambridge University in England. Bentley has practiced criminal and labour law, with the firm of Bentley & LeRoy LLP. In 1985, he was a founder and first chairperson of Neighbourhood Legal Services London & Middlesex, a legal clinic intended to assist low-income clients.
Since 1992, Bentley has worked as a part-time teacher at Western. He helped establish the university's Law School Careers Office, has lectured at various continuing education courses. In 2000, he published a book entitled Criminal practice manual: A practical guide to handling criminal cases.