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By David Ljunggren
OTTAWA, June 12 (Reuters) - Chief Justice Beverley McLachlin, the first woman to become Canada's top judge, will step down this December after serving a record 18 years in the job, the Supreme Court said on Monday.
McLachlin, 73, presided over a series of landmark rulings on topics such as same-sex marriage and prostitution and clashed on occasion with the former Conservative government of Prime Minister Stephen Harper, which felt the court had become too activist.
"She has been a judicial leader and trailblazer for almost four decades ... Her contributions reach into every part of our law," Liberal Prime Minister Justin Trudeau said in a statement.
The court declined to comment when asked about the reasons for her retirement.
McLachlin joined the top court as a judge in 1989 and became chief justice in January 2000. She would have reached the mandatory retirement date of 75 in September 2018.
"It's with sadness that we received her retirement ... She has helped us as a country define and advance and develop the law," Justice Minister Jody Wilson-Raybould told reporters.
Under McLachlin, the court allowed same-sex marriage and supervised injection drug clinics and physician-assisted suicide. It also struck down major restrictions on prostitution and expanded the rights of Canada's marginalized aboriginal population.
"She's been involved in some of the most seminal legal decisions of our time," said Bill Flanagan, dean of law at Queen's University in Kingston.
In 2014, under McLachlin, the justices barred a Harper nominee from taking a job at the Supreme Court, citing the candidate's lack of qualifications. The episode triggered a rare public dispute between the prime minister and chief justice.
"She managed herself with a great deal of dignity and the legal profession ... was very strongly in support of her," Flanagan said by phone.
At a rare news conference in August 2015, she dismissed the idea the court had become too activist.
"We try to answer the questions put before us in accordance with the constitution and the law. I leave the labels to other people," she said.