Frank has had a distinguished career in private practice, academia, government and the judiciary. He began his career in 1964 as a lawyer at Dewey, Ballantine, Bushby, Palmer & Wood in New York, where he practised corporate and securities law. In 1967, he joined the Faculty of Law, University of Toronto, and was a professor of law there until 1985. He also served as Vice-President, Internal Affairs at the University of Toronto from 1975 to 1979 and Dean of the Faculty of Law from 1979 to 1983. From 1983 to 1985, he was Vice-President and Provost of the University. In 1985, Frank was appointed Deputy Minister of Justice and Deputy Attorney General for Canada; in 1988, Chief Justice of the Federal Court of Canada; and in 1991, a Justice of the Supreme Court of Canada.
During his career, Frank was also a Commissioner of the Ontario Securities Commission from 1982 to 1985, and Counsel to the Estey Commission in 1974.
In June 2005, Frank was appointed by the Government of Canada as its representative to lead discussions toward a fair and lasting resolution of the legacy of Indian residential schools. In November 2006, Frank was appointed by the Ontario Ministry of the Attorney General as a member of the Board of Governors of the Law Commission of Ontario. He currently serves as Chairman of the Higher Education Quality Council of Ontario. In December 2006, he was appointed Commissioner to lead an inquiry into the conduct of Canadian officials regarding certain individuals.