Canada’s prime minister, Mark Carney, has appointed a former supreme court justice and war crimes prosecutor as the country’s new governor general, saying her appointment would reflect the importance of global institutions.

Louise Arbour, a celebrated jurist, served as United Nations commissioner and prosecuted war crimes in the former Yugoslavia and Rwanda, will serve as Canada’s representative of King Charles III.

“Louise Arbour will represent the best of Canada to Canadians and to the world. A country that’s a bastion of security, prosperity and justice – a beacon to a world lost at sea. A Canada that is clear-eyed about the challenges we face and steadfast in the values we uphold,” Carney said in Ottawa as he announced his pick on Tuesday.

Arbour will succeed Mary Simon, who became the first Indigenous person to hold the title when she was appointed by then prime minister Justin Trudeau in 2021.

At age 79, Arbour is the oldest person to be appointed to the role. Born in Montreal, Arbour taught law before she was appointed first as a judge in the province of Ontario and later to the country’s supreme court in 1999. She retired in 2004.

Her later work outside Canada received substantial acclaim, both with the United Nations high commissioner for human rights and developing global migration policy. Arbour also served as chief prosecutor in two high-profile international criminal tribunals, which investigated the allegation of war crimes committed in Rwanda and the former Yugoslavia. With the latter, Arbour’s work led to the indictment of the Serbian president, Slobodan Milošević – the first time an incumbent leader had faced justice from an international tribunal.

Carney praised Arbour as a figure who reflected the importance of institutions – a theme he has repeated in speeches to global audiences.

“The conviction that institutions are the load-bearing walls of a civil society – and that they remain trustworthy only as long as someone is willing to hold them accountable,” he said. “Louise Arbour has held nearly every office a Canadian jurist can hold, and several that no Canadian had held before.”

The role of governor general combines ceremonial and constitutional duties. During her five-year term as vice-regal, she is also the commander-in-chief of the Canadian armed forces. In 2021, Arbour was tapped to lead an independent review of Canada’s military, where she recommended reforms to address sexual misconduct and institutional culture.

“The measure of her career is neither in the office she has held nor the awards she has received. It is in the lives she has changed through her service,” said Carney.

Simon, the outgoing governor general, has focused her tenure on Indigenous reconciliation and environmental causes. She hosted visits by prominent leaders, including the king and Pope Francis. As part of the visit, the spiritual leader of the Catholic church apologized to survivors of Canada’s residential school system.

But Simon, whose first language is Inuktitut, struggled with French – a language that has immense political weight in Canada.

As a result, the prime minister was under pressure to find a candidate fluent in both English and French.

Arbour told attenders she accepted the role “with a deep sense of duty”.

“The strength of this country resides in stable institutions managed with wisdom and sustained through the desire for the wellbeing of our country and the planet,” she said. “I will be the representative of the crown in a constitutional arrangement that has served Canada extremely well throughout our history, even more in recent decades. A system that will continue to provide continuity in our institutions and form of governance,” she said.