He practised law in Kingston from 1979 to 1982 and was a sessional lecturer in
civil procedure at the
Queen's Law School from 1980 to 1982. He was a professor of law at
Dalhousie University from 1982 to 1992 and again from 1995 to 1997. In that role, Cromwell decided in favour of recognizing the claimant's
Aboriginal title in
R v Marshall; R v Bernard, but his reasoning was overturned by the Supreme Court. However, this process was bypassed when Cromwell was officially appointed; Prime Minister Harper had expressed concern that waiting for the committee to meet would hobble the court in executing "its vital constitutional mandate effectively." However, Parliament wound up meeting only very seldom after Cromwell's initial nomination, mainly because of a federal election called by Harper for October 14, and then the subsequent proroguing of Parliament on December 4, until its scheduled resumption on January 26, 2009. While on the Supreme Court, Cromwell joined the rest of the court in a unanimous decision in ''
Tsilhqot'in Nation v British Columbia'' to recognize the existence of Aboriginal title for the
Tsilhqot'in. The decision that drew in large part from his previously rejected Nova Scotia Court of Appeal ruling in
R v Marshall; R v Bernard, vindicating his earlier judgement. Justice Cromwell retired from the Supreme Court of Canada on September 1, 2016, at the age of 64, fulfilling a personal intention to resign around age 65. Throughout his eight-year service on the Supreme Court, Cromwell authored around a hundred decisions and earned a reputation as a centrist, neither left or right, activist or deferent. On February 12, 2019, former
Minister of Justice and Attorney General of Canada Jody Wilson-Raybould retained Cromwell to advise her in the wake of her resignation from the federal cabinet. In December 2020, Cromwell was commissioned by the
University of Toronto Faculty of Law to conduct an impartial investigation of the search process it had used to hire a new director for its International Human Rights Program. The report was commissioned after allegations that a sitting judge of the
Tax Court of Canada, David Spiro, had improperly interfered with the search process. On March 15, 2021 Cromwell released his report. The Report has drawn criticism from legal scholars and academics as to its accuracy, scope, and potential conflict of interest issues. The scandal and subsequent Report has led to the
Canadian Association of University Teachers censuring the University of Toronto. ==See also==