Both the president and the first lady were also photographed at Lancaster House, signing a book of condolences.
Other distinguished heads of states and dignitaries arrived to greet King Charles III, who along with other members of the royal family, would be welcomed with a reception.
Also in attendance was the first lady of Ukraine, Olena Zelenska, who paid her respects to the late monarch amid the ongoing war in her country.
The prime minister of the United Kingdom and Boris Johnson's successor, Liz Truss, was photographed meeting King Charles earlier in the day. She hosted several leaders in a conversation of her own, including Canada's prime minister, Justin Trudeau.