Mark Carney, the former governor of Canada and the UKs central banks, has won the race to succeed Justin Trudeau as Canadas prime minister and Liberal Party leader.
Carney, 59, who beat three rivals in a landslide, will lead the Liberals into the next general election, which is expected to be called in the coming weeks.
In his victory speech, Carney lashed out at US President Donald Trump, who has ignited a trade war with Canada and said he wants to make the country the 51st US state. "These are dark days brought on by a country we can no longer trust," he said.
Carney, who has never served in elected office, takes power at a time of deep instability for Canada.
The leadership race began in January after Trudeau resigned following nearly a decade in office. He had faced internal pressure to quit over deep unpopularity with voters.
Carney won on the first ballot on Sunday evening, taking a decisive 85.9% of the vote to beat his nearest rival, former Finance Minister Chrystia Freeland.
Loud cheers erupted as the results were announced to a crowd of some 1,600 party faithful in Ottawa, Canadas capital.
He is expected to be sworn in as prime minister in the coming days, and will lead a minority government in parliament.
He could either call a snap general election himself or the opposition parties could force one with a no-confidence vote later this month.
The Liberals trailed the Conservatives, led by Pierre Poilievre, at the beginning of this year by more than 20 points.
But they have narrowed the gap since the trade war with Trump began and are now statistically tied with the official opposition, according to some polls.
Many of Sundays speeches focused on the Trump administrations threats to impose what Carney described as "unjustified tariffs" on Canada, Americas largest trading partner.
"Hes attacking Canadian workers, families, and businesses," said Carney, sparking loud boos from the crowd. "We cant let him succeed."
He said his government would keep tariffs on US imports "until the Americans show us respect".
"I know these are dark days," Carney said. "Dark days brought on by a country we can no longer trust.
"Were getting over the shock, but let us never forget the lessons: we have to look after ourselves and we have to look out for each other. We need to pull together in the tough days ahead."
Carney also pledged to "secure our borders", which has been a key demand of Trump in their tariff stand-off.
The US president even got a mention in Carneys attacks on his main opponent, Conservative leader Poilievre.
"Pierre Poilievres plan will leave us divided and ready to be conquered," said Carney.
"Because a person who worships at the altar of Donald Trump will kneel before him, not stand up to him."
The Conservatives have had to pivot politically since Trudeaus resignation, and are targeting Carney as "just like Justin", while accusing the Liberals of a "sneaky" plan to win a fourth term by simply substituting their leader.
Poilievres party has also accused Carney of lying about his role in moving investment firm Brookfield Asset Managements head office from Toronto to New York, though Carney says the formal decision to relocate the firm was made after he quit the board.
Federal Public Safety Minister David McGuinty, who endorsed Carney, told the BBC that he "embodies the kind of quiet determination, but steely determination and competence to deal with some of these big issues".
"Im really, really excited for whats coming. And frankly, its time for an election."
The former central banker has run on a broadly centrist agenda, a shift away from Trudeau, who shifted the Liberals to the left.
Trudeau gave his final speech as prime minister on Sunday night, warning that Canada was facing an "existential challenge" from the US under Trump.
Carney praised his predecessors "strength and compassion as a fighter for Canada".
Among the new Liberal leaders key promises is to push forward on major energy projects like pipelines, which have faced political roadblocks in recent years.
He has promised major investments in housing and clean energy projects, and to liberalise trade within Canada, where barriers remain between provinces, and to diversify trade away from the US.
During the leadership race, Carney promised to cap the size of the federal government, which expanded 40% under Trudeau and to undertake a programme review.
He said in late February he would run a "small" deficit for three years "that aligns with our fiscal capacity", before balancing the operational budget.
Carney has said the country needs significant investments to counter Trumps tariff threats, including in defence, energy and port and rail infrastructure.