Prime Minister Justin Trudeau will have his first chance to shape the country's highest court this summer, with the surprise announcement that Supreme Court Justice Thomas Cromwell will retire on Sept. 1.

Replacing Justice Cromwell, a centrist, is also an opportunity for Mr. Trudeau to put in place a new appointment process, after his predecessor, Stephen Harper, ended Parliament's role in the selection of Supreme Court judges and scrapped a public hearing after the appointment. The last three judges were chosen in a secretive process from which Parliament was excluded for the first time in more than a decade, with only brief announcements from the Prime Minister about why he selected them.

But the court's coming vacancy will also mean a political headache. Mr. Trudeau has promised he will appoint functionally bilingual judges to the Supreme Court. That will reduce the pool of candidates. By convention, the judge needs to be from Atlantic Canada to replace Justice Cromwell, who is from Nova Scotia. Newfoundland and Labrador has never had a judge on the Supreme Court, and a 2011 study found it had no bilingual judges on its appeal court, the place from which most Supreme Court judges are drawn.

Story continues below advertisement

At just 63, Justice Cromwell has been on the court for seven years, making him its third-longest serving member. The court's mandatory retirement age is 75. A soft-spoken man with an unassuming manner, he was considered one of its leading lights, and a possible replacement for Chief Justice Beverley McLachlin, whose mandatory retirement date is in 2018. He was also the kind of judge some view as the ideal – a man with both conservative and liberal elements in his thinking who could not be easily pigeonholed.

"He was someone born to be a judge," Wayne MacKay, a Dalhousie University law professor and former colleague, said. "He's a nice mix of the rational … and a sensitivity to the actual situation. You don't want someone who is a rational machine; you want someone who puts together the rational and the more emotive side of judging. And I think he does this really well."

The last three appointments have been a throwback to older, less transparent times. A Liberal justice minister, Irwin Cotler, first created a committee in 2004 to winnow down a long list of candidates to a short list, from which the prime minister would choose. In 2005, Mr. Cotler created a parliamentary hearing in which he explained his choice for the court. The following year, Mr. Harper introduced unprecedented transparency by putting the new judge in front of a televised hearing of parliamentarians.

But two years ago, he cancelled both the parliamentary selection committee and the public hearing, after The Globe revealed the secret list of candidates from which he selected Marc Nadon, who was ultimately rejected as legally unqualified by the Supreme Court. Mr. Trudeau's mandate letter for Justice Minister Jody Wilson-Raybould instructs her to ensure the process is transparent, inclusive and accountable.

Justice Cromwell wrote for the majority in a 2015 case, M.M. v the Minister of Justice, ordering a mother extradited to the United States to face child-abduction charges though the children had run to her from their violent father. The minority judges called that "Kafkaesque." He was in the majority in R v. Nur, striking down a three-year minimum sentence for illegal gun possession, which the minority judges suggested was an unacceptable form of judicial activism.

In R v. Fearon, also in 2014, he wrote for the majority in a 4-3 ruling that police may search cellphones without a warrant when they make an arrest. In another 2014 case, R v. Spencer, he wrote for a unanimous court that police needed a judge's permission to ask Internet providers for basic information on their customers – including those suspected of being involved in child pornography.

"He did just about everything from A to Z, from aboriginal to zoning," Ottawa lawyer Eugene Meehan, a partner in a firm specializing in the Supreme Court, said.

Story continues below advertisement

Justice Cromwell also is chair of a committee trying to improve people's access to the courts. Sébastien Grammond, a law professor at the University of Ottawa, called him "a great legal mind, very rigorous, yet at the same time compassionate, progressive – but within a system of precedent. [Lawyers appearing in front of him] cannot argue things that are too … creative."