He was called to the Bar of Manitoba in 1966. Also in 1966, he married Sheila Dorfman, a Montreal doctor. They have four children: Ronald, Douglas, Tracey and Robert. Marshall Rothstein practiced law primarily in the fields of transportation and competition law and was a partner with the Winnipeg law firm of Aikins, MacAulay & Thorvaldson. From 1970 to 1992, he was a lecturer in transportation law at the University of Manitoba. In 1992, he was appointed to the Federal Court Trial Division, ex officio of the Court of Appeal, and appointed to the Court Martial Appeal Court of Canada. In 1999, he was appointed a Judge of the Federal Court of Canada, Appeal Division. He wrote 578 judgments for the Federal Court and 324 judgments for the Federal Court of Appeal. In 2017, he was appointed a Companion of the Order of Canada by Governor General David Johnston for "his eminent service as a jurist, notably on the Supreme Court of Canada, and for his dedication to legal education." Rothstein was one of the candidates (the others being Peter MacKinnon and Constance Hunt) recommended by a committee convened by the outgoing Liberal government to be appointed to the Supreme Court of Canada, following John C. Major's retirement from the bench in early 2006. Prime Minister Stephen Harper chose Rothstein for the Governor General to appoint to the top court. Rothstein's appointment by the Conservative government was criticized because of his unilingualism. He was the only justice of the Supreme Court who was not bilingual, prior to the 2011 appointment of Justice Michael Moldaver. Many Canadian conservatives had long been critical of the process of appointing judges to the Supreme Court of Canada, wherein the Prime Minister is the sole advisor of the Governor General in the matter, and though he or she consults legal experts, no input is given from other (especially opposition) politicians. Conservative Leader Stephen Harper had thus made a promise to "reform" the appointment process during previous elections. Shortly after Prime Minister Harper put Rothstein's name forward, Harper acted on his promise to create an Ad Hoc Committee to Review a Nominee for the Supreme Court of Canada, a new creation intended to allow the nominee to face questioning by members of the Canadian Parliament, similar in spirit to the Senate judicial hearings that occur as part of the Supreme Court appointment process in the United States. The Panel was chaired by constitutional law professor Peter Hogg. The new procedures replaced a reformed appointment process introduced by the previous Liberal government, but which had not yet been applied.