Prince William met the leader of the free world after President Joe Biden made a detour to say hello, ahead of his Earthshot Prize awards.
Royal aides only discovered in recent days that the US leader would be in Boston at the same time, and were delighted when the White House said he would like to make time to speak in person.
As the royal already had an engagement at the John F Kennedy Presidential Library and Museum with Caroline Kennedy, JFK’s only surviving child, President Biden even offered to come to him.
The two men met in a picturesque spot outside the library at Columbia Point, overlooking Boston Harbour, under crisp, bright but cold winter skies.
In the distance, a band of protestors could be heard shouting indecipherable slogans regarding the President.
He arrived in an extensive convoy before making his way down to the harbourside on foot, remarking to journalists: ‘It’s a bit cold, right.’
He was invited to stand on an ‘x’ marked out on the ground and waited patiently for a few seconds before William walked down from the library where he had just had lunch with Ambassador Kennedy and representatives from the Earthshot Prize.
"Where’s your top coat?," he called out to the prince as he approached him. William - wearing just his suit - laughed and smiling.
The two men shook hands, with Biden clasping onto the prince’s as they spoke about the "spectacular setting".
Biden then put his hands in his pocket as William spoke and gestured expressively.
After around two minutes they were invited to make their way into the library for private talks, walking by the harbour railing to take in the view.
It is understood that President Biden had juggled his diary to make around half an hour to spare.
Earlier a spokesman for the Prince of Wales said he had been looking forward to meeting the President at the JFK library.
He added: "During the meeting the Prince of Wales is likely to discuss tonight's Earthshot Prize ceremony, taking place at the MGM Music Hall.
"The Prince will share the legacy of last year's finalists and the impact they're having globally.
"He's also keen to express his thanks to the American people for the warm reception he and the Princess have received this week."
Earlier today they were pictured behind the scenes during rehearsals for the ceremony, having been welcomed by roaring crowds, including a boy dressed as a Buckingham Palace guard.
When the royal pair touched down in the States on Wednesday, the White House had confirmed it was "finalising the details" of the meeting, while a Kensington Palace spokesperson said similar.
William and Kate - who are undertaking their first royal foreign visit since the death of the Queen - landed at Logan International Airport.
The heir-to-the-throne and his wife travelled on a commercial British Airways flight.
He paid tribute to his late grandmother during a brief speech in which he described the monarch as "one of life's optimists".
Queen Elizabeth II had also visited the US city in 1976 and William said she remembered it with "great fondness".
More than a dozen Boston landmarks were lit up green that evening, including the City Hall, John F. Kennedy Presidential Library and Museum, in tribute to the visit.
The couple have also enjoyed an NBA basketball match between the Boston Celtics and Miami Heat game at TD Garden.
William's environmental awards will see singer Billie Eilish lead a stellar entertainment line-up during a ceremony - dubbed the prince's "world cup" moment - that aims to celebrate and scale up environmental solutions to repair the planet.
The three-day trip has been overshadowed by the race row that saw William's godmother Lady Susan Hussey resign as a royal aide after she repeatedly questioned a prominent black British-born domestic abuse charity boss about where she "really came from".
A trailer promoting the Duke and Duchess of Sussex's Netflix docuseries was released on Thursday, with the timing interpreted by some royal commentators as a snub to William and Kate.
The Waleses are said to have “no plans” to catch up with Harry and Meghan, despite putting their differences aside together to appear publicly outside Windsor after the Queen's death in September.
In a black-and-white image released of Kate she is seen in silhouette walking across the stage of the theatre as an image of a cloud-filled sky is projected behind the royal.
In another image William is photographed laughing as he chats to someone off camera with broadcast equipment in the background of the picture.
