He was appointed one of Her Majesty's Counsel (December 1972) and Solicitor General of New South Wales (1974–1978). He retired early from this appointment and returned to private practice at the Bar, where he formed and headed a floor of Barristers in Garfield Barwick Chambers. Marr was lecturer for New South Wales Bar Council on "Conduct of Criminal Trials" (1974–1986) and author of the work of the same name, currently in use for Bar Council lectures. In 1998, Marr was awarded a Medal of the Order of Australia in recognition of his services to veterans' affairs and the Christian Brothers College, Waverley. After the war Marr enlisted into the Australian Army Legal Corps, and was a Consultant to director, Army Legal Services, Australian Army Legal Corps with the rank of colonel. Marr was Honorary Colonel of the Waverley College Cadet Unit until his death on 16 November 1999, a period in excess of 40 years.