Leonard has been practicing criminal law for over fifty years. In 1974, he was one of the founding members of the Defence Counsel Association of Ottawa and served as its first President. He was appointed Queen’s Counsel (QC) in 1980. Throughout his career, Leonard has practiced before all levels of the Ontario and Federal Courts.

In addition to his impressive courtroom record, Leonard has appeared before various federal judicial inquiries, including the Gomery Inquiry and Arar Inquiry. He served as amicus curiae to the Federal Court in matters involving Canada’s anti-terrorism legislation and was part of the first group of lawyers to be named Special Advocates by the Minister of Justice and Attorney General of Canada shortly after the legislation was passed.

Leonard has also provided instruction in the Criminal Procedure Program at the Law Society of Upper Canada’s Bar Admission Course Program. For several years, he served as the program’s senior instructor.