Prime Minister Justin Trudeau has selected a politically connected Toronto corporate consultant and a high-powered lawyer from Quebec to take over as Canada's envoys to the United States and the United Nations.

David MacNaughton, chairman of Strategy Corp., who co-chaired Mr. Trudeau's Liberal election campaign in Ontario, will become ambassador to Washington with a mandate to improve Canada-U.S. relations, The Globe and Mail has learned.

Marc-André Blanchard, the chief executive officer of the law firm McCarthy Tétrault, who was on Mr. Trudeau's transition team, will be named ambassador to the United Nations. His instructions are to reassert Canada's role at the UN.

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A senior federal official speaking on background said the Prime Minister chose the two men for their vast business and political experience and close personal connection to Mr. Trudeau.

"It came down to trust. You have to have someone in these roles that you will trust with your life," the senior official said. "And people who deal with the government need to know they have the Prime Minister's complete confidence."

The official said Mr. Trudeau tapped Mr. MacNaughton because of his extensive experience working in the United States and profound understanding of how Washington operates.

For many years, Mr. MacNaughton worked in New York as chair of Hill & Knowlton, one of the world's most influential consulting firms. He spent much of his time in Washington lobbying political figures and senior government officials.

The official said Mr. MacNaughton will be aided by his spouse and business partner, Leslie Noble, an influential figure within the Ontario Progressive Conservative Party, who can reach out to the Republican-controlled Congress.

"Regardless of who wins the presidency, Congress will be run by the Republicans, and Leslie will be able to help in making the case for Canada on Capitol Hill," the official said.

Gordon Giffin, a former U.S. ambassador to Canada, praised the appointment of Mr. MacNaughton and said he has what it takes to be a first-rate envoy.

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"He brings an adult lifetime of studying and working in the public policy arena and the job of an ambassador is to affect public policy," Mr. Giffin said.

"Washington is probably the most political city on the planet, and understanding politics, how the legislative body and executive reach conclusions, is awful important, so his background and history will be enormously advantageous."

Mr. MacNaughton replaces former NDP Manitoba premier Gary Doer, while Mr. Blanchard takes over from career diplomat Guillermo Rishchynski at the UN mission.

The first major responsibility of Mr. MacNaughton is to help arrange Mr. Trudeau's first bilateral visit to Washington, which will include a state dinner at the White House, the first time in 19 years that a Canadian prime minister has received that honour.

Relations between President Barack Obama and Conservative prime minister Stephen Harper were cool, particularly over the proposed Keystone XL oil pipeline.

Mr. Harper was said to have annoyed the White House when he told a New York audience approving the project was a "no brainer" and noted support for it in the Republican-controlled Congress.

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There were also disputes over economic protectionist measures and who should pay for a U.S. customs plaza at the Windsor-Detroit bridge.

In the last election, key Democratic players in Mr. Obama's inner circle were advisers to the Liberal campaign on advertising and social media.

The embassy in Washington is Canada's most important diplomatic mission. Mr. Trudeau campaigned on improving relations with the White House, taking action to slow climate change in North America, and easing the free flow of people and goods across the border.

Derek Burney, an ambassador to Washington under Brian Mulroney, said the best ambassadors are those who have a personal relationship with the prime minister.

"It's important that he or she be known in Washington or Ottawa as someone who has the direct ear of the prime minister, because nothing is more valuable in Washington than that as a factor of influence and relevance. Frankly, it's almost as relevant for influence here. It's very important that the bureaucracy know this is an ambassador who has the ear of the prime minister," Mr. Burney said.

Former Quebec premier Jean Charest, a personal friend and legal associate of Mr. Blanchard, said Mr. Trudeau is sending a message that Canada intends to re-engage at the United Nations.

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"The image the new government wants to present is that Canada is going to be much more of a presence in international forums than it was in previous years," Mr. Charest said.

"Marc-André has the skill set that will serve him well in that kind of job, in dealing with people who are in a forum where relationships count, where judgment is a premium."

Mr. Harper often skipped annual meetings of the UN General Assembly, criticized the international body, and appeared unconcerned when Canada failed to win a seat on the UN Security Council in 2010.

"At the annual opening of the United Nations, Mr. Harper is remembered for being at Tim Hortons, and that's not the image we want to be of Canada," Mr. Charest said.

Mr. Trudeau promised during the election campaign to seek a spot for Canada at the Security Council when one of the 10 temporary seats opens up.

Bernard Lord, the former Conservative premier of New Brunswick who worked with Mr. Blanchard at his law firm, said he has the ability to fulfill that objective.

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"He's a hard worker. He gets along with people. He gets things done. He is a strong communicator, great interpersonal skills. He will do everything he can to represent us as well," Mr. Lord said.