The US President stopped off in Britain on his way to the Nato summit which starts on Tuesday in Lithuania, after being invited by King Charles when he could not attend the coronation.
They discussed “issues of mutual interest and concern” during their 20-minute meeting, according to Buckingham Palace – and also heard assessments from the politicians overseeing climate policy in the US and UK.
The King has shown an interest in the environment for decades, courting controversy when he was Prince of Wales by intervening in controversial political issues including by sending memos to ministers setting out his views.
While he has promised to avoid stoking political divisions since taking the throne last year, he nonetheless put the climate at the centre of his talks with Mr Biden.
The meeting coincided with a gathering of the Climate Finance Mobilisation Forum, also at Windsor Castle, where the leaders of large financial firms such as HSBC, NatWest and BlackRock discussed ways to ramp up investment in technology that will reduce carbon emissions.
It was overseen by Grant Shapps, the Energy Security and Net Zero Secretary, and US climate envoy, John Kerry. According to ABC News, Mr Kerry said: “There’s a full understanding here, particularly with the evidence of the last few weeks piling on, that when scientists are telling us they are terrified by what they’re seeing, and when we hear those same scientists telling us we are in uncharted territory, this group has come together to try to figure out, ‘OK how do we deploy the funds necessary to invest to create the new clean energy economy.'”
A Buckingham Palace spokesman said the King and Mr Biden had had “an extremely cordial and successful meeting that touched on a wide variety of issues of mutual interest and concern”, adding: “You will have seen for yourselves the personal warmth between His Majesty and the President.”
The two men met for 20 minutes in a private sitting room at Windsor Castle and inspected items from the Royal Collection with a link to the US.
King Charles met Mr Biden outside the castle after he landed by helicopter following his talks at 10 Downing Street with Rishi Sunak.
The leaders inspected a Guard of Honour formed of The Prince of Wales’s Company from the Welsh Guards, then watched them march off before they sat down for tea.
The last time the President saw the monarch was when he came to London for the state funeral of the late Queen, whom he had met several times during his decades-long career as a senator and vice president.
He sent his wife, Jill Biden, to represent him at the coronation in May but at the same time promised to visit the King at a later date.
Monday’s trip fell short of a full state visit, although Mr Biden may be invited back for a longer series of events including a full white-tie dinner in the future.