The Duke of Kent, the Queen's cousin, joined her on a dais in the castle's quadrangle in his role as Colonel of the Scots Guards today.
The first lady spent time with the Queen's granddaughter-in-law, attending a school with her in Cornwall on Friday, where the pair met some of the pupils in reception class, before sitting on a round table about education and Early Years development.
But this being the Royal family, such visits for afternoon tea include a military display and some fabulous big band tunes, including the Star Spangled Banner, after which the president was invited to inspect the Guard of Honour.
The Queen and the first lady waited as the president inspected the Guard of Honour.
"I said we could fit the White House in the courtyard", the president disclosed to reporters waiting on the Heathrow tarmac for news of the conversation.
They met briefly on Friday evening in Cornwall, the first time the Bidens had met the Queen despite his eight years as vice president when he worked with Barack Obama.
Biden, who first met the queen in 1982 as a Democratic senator for DE, said following the meeting that she was "extremely gracious" and that she reminded him of his mother in terms of "the look of her and the generosity".
The trio enjoyed a Guard of Honour formed of The Queen's Company First Battalion Grenadier Guards, and the USA national anthem was played beneath blue skies and hot sunshine. Biden returned to the dais and he, the first lady and the white-gloved woman who has held the British throne for almost 70 years watched the military march past before they went inside for tea. The US First Lady, who is set to return to Washington after the regal reception, while her husband flies on to Europe for further meetings, said Friday it was "an exciting part" of their United Kingdom visit. She was very generous.
The monarch has now received four presidents at Windsor, with the Bidens following the Trumps in 2018, the Obamas in 2016, President George W. Bush and his wife in 2008 and the Reagans in 1982.
During her 69 years on the British throne, the longest reign of any English monarch, there have been 14 US presidents, of which Elizabeth has met all bar Lyndon Johnson.
When Dr Biden was quizzed on whether she asked the Duchess of Cambridge for advice on meeting the Queen, she brushed off the suggestion, saying they were "busy".
It was due to last around half-an-hour, but overran by nearly 20 minutes.
The Queen beamed in the sunshine, tapping her feet to the music, as she celebrated her official birthday with a scaled back military procession at Windsor Castle.