OTTAWA – Canada's ambassador to the United States, David MacNaughton, is resigning at the end of August to pursue a career in the private sector. In announcing his departure MacNaughton said that while his three years in the job were the most challenging of his life, he wouldn't trade the experience for anything.

During a press conference on Thursday afternoon at the embassy in D.C. about his coming resignation, MacNaughton said despite some "bumps along the way," he thinks the Canada-U.S. relationship is "stronger than it's ever been."

In a statement confirming the high-profile ambassador's departure from Canada's foreign service, Prime Minister Justin Trudeau said that: "It is with great affection and gratitude – and of course, considerable regret – that I have accepted Ambassador David MacNaughton's decision to leave Washington. D.C., at summer's end, to return to his home in Toronto, and take up new challenges."

The prime minister said that MacNaughton "remains a trusted advisor, friend, and counselor," and thanked him for his "tireless" and "critical" work. Sources have told CTV News that MacNaughton will be willing to help out with the Liberal's reelection campaign if asked.

"I am available to the prime minister and to my friends and colleagues to provide advice and counsel whenever they want to seek it. I have indicated to them that I am quite prepared to," MacNaughton told reporters. "I have never been very shy about offering my opinions, so if they are called on, I will offer them."

In a statement of his own, the outgoing top Canadian diplomat in the U.S. said that his decision to leave was “bittersweet,” and one he did not take lightly. Though, MacNaughton said that he had “long planned” to leave the job ahead of the Oct. 21 federal election, in part to spend more time with his family.

MacNaughton has held the position since March 2016 and was a central figure during the NAFTA renegotiation process and efforts to lift the steel and aluminum tariffs that the Americans imposed during the talks.

Following that intense stretch of frosty relations between the two countries, MacNaughton has remained Canada's main contact on the ground in Washington, D.C. with Congress and U.S. President Donald Trump's administration on ratification of the new deal, as well as on other pressing issues including border security and ongoing tensions with China.

MacNaughton said that while challenges remain on the Canada-U.S. trade file, he still expects that the new NAFTA will pass in the U.S. and Canada, in the coming months.

With Parliament adjourned and the U.S. House of Representatives summer session concluded without signing off on the agreement, it's looking highly unlikely that the renegotiated text will be formally in place until after the fall election in Canada. Mexico has already ratified the deal but all three countries need to do so in order for it to become active.

MacNaughton said despite Democrats wanting changes to the agreement he expects it will pass as-is, throwing out the possibility of clarifying side letters as one way to appease those concerns without reopening the agreement.

"We succeeded at the NAFTA table because of personal diplomacy, across the border and across the political spectrum, both here and in the U.S. And because of the unrelenting effort of literally hundreds of dedicated Canadian public servants, including my own staff here in Washington," he said in his departing statement.

MacNaughton called the job the "greatest honour" of his life and offered two parting pieces of advice: "We absolutely must never rest on our laurels – but rather always seek to push further into new markets and new opportunities, around the world," and "the preservation of our most important relationship, with our neighbour, friend and ally, the United States… will always depend, as they did in the NAFTA talks, on personal contact and relationships."

Prior to his appointment as ambassador, MacNaughton co-chaired the 2015 Trudeau Liberal election campaign in Ontario, worked for past provincial Liberal leaders and was involved in government and public relations work.

In a tweet, United States Trade Representative Robert Lighthizer wished his friend well, saying that MacNaughton is a "highly skilled negotiator and diplomat."

Bruce Heyman, the former U.S. ambassador under Barack Obama said that one of the best parts of his position was working with MacNaughton. "He is a consummate professional and has been a wonderfully successful representative of Canada," Heyman said in a tweet.

Kirsten Hillman will become Canada's acting ambassador to the U.S. upon MacNaughton’s departure. Hillman has been Canada's deputy ambassador since 2017 and prior to that was the assistant deputy minister of the trade agreements and negotiations branch at Global Affairs Canada.

"Canada will continue to be in strong hands in Washington," Trudeau said.