To world leaders, the 81-year-old Arizona Republican who lost his year-long battle with brain cancer this weekend was a "hero", an "inspiration to millions", and a man "of great courage".
McCain stopped receiving cancer treatment last week, his family saying "the progress of disease and the inexorable advance of age render their verdict". "My family and I send prayers for Cindy and the McCain family".
But in his address to the US Studies Centre a year ago, Senator McCain did not shy away from conceding that Mr Trump was "unsettling" American allies, or that there is a debate underway in the United States about its role in the world, its resolution uncertain.
President Donald Trump is offering his "deepest sympathies and respect" to McCain's family.
The late Arizona Republican, had organised his funeral, and close associates had told the White House in May that he did not want Trump to be invited.
In the upper chamber of Congress, McCain established himself as a leading voice on national security and foreign policy, particularly from his chairmanship of the powerful Armed Services Committee. "He showed us that boundless patriotism and self-sacrifice are not outdated concepts or cliches, but the building blocks of an extraordinary life".
News of Mr McCain's passing has elicited an outpouring of tribute and sympathy from Americans across the political aisle.
He said despite their differences, they shared a "fidelity to something higher - the ideals for which generations of Americans and immigrants alike have fought, marched, and sacrificed".
"John McCain's life is proof that some truths are timeless", he said in a statement.
Former President George W Bush, who defeated Mr McCain for the 2000 Republican presidential nomination, called his one-time political rival "man of deep conviction and a patriot of the highest order" and a "friend whom I'll deeply miss".
In a joint statement, the former USA leader and ex-secretary of state said: "Senator John McCain believed that every citizen has a responsibility to make something of the freedoms given by our constitution, and from his heroic service in the navy to his 35 years in Congress, he lived by his creed every day".
She continued: "He was a tough politician, a trusted colleague, and there will simply never be another like him".
The secretary of defence said the U.S. had "lost a man who steadfastly represented the best ideals of our country".
"I will always be especially grateful for his leadership in our successful efforts to normalise relations with Vietnam".
Following the announcement of his death on Saturday evening, U.S. president Donald Trump said his "deepest sympathies" went to Mr McCain's family.
He missed a White House ceremony on December 12, 2017, in which President Donald Trump signed the annual defense bill into law - one of McCain's signature achievements.
French President Emmanuel Macron tweeted in English that McCain "was a true American hero".
Cindy, his wife, thanked all those who offered their support, adding that her heart is "broken".
Former prime minister Ehud Barak said that McCain, a longtime personal friend, was among the few principled political leaders today.
"Another anti-China warrior is down, godspeed, you can finally rest, there's no China in heaven", wrote one user on the Twitter-like Weibo.
"There will never be another John McCain".
He was diagnosed with an aggressive brain tumour in July 2017 during surgery to remove a blood clot from above his left eye, and had been undergoing medical treatment.