The social call took place one day after Queen Elizabeth's 90th birthday. The president and first lady presented the queen with a photo album featuring moments from her previous visits with U.S. presidents and first ladies.
The Obamas, Queen Elizabeth and Prince Philip enjoyed lunch in a private dining room at the castle. Michelle Obama wore a fuchsia Oscar de la Renta dress, while the queen chose a periwinkle blue ensemble with floral headscarf.
President Obama held a joint press conference with British Prime Minister David Cameron after lunch, calling the queen a "jewel to the world." Cameron lauded Obama as a friend to the U.K. following the president's urging Britain to remain in the European Union.
The Obamas will have dinner with the queen's son Prince William and Kate Middleton, Duchess of Cambridge, Friday evening. Prince William and Middleton will host the couple at Kensington Palace, also outside London.
"Their Royal Highnesses are very much looking forward to welcoming the President and First Lady to the Palace," Kensington Palace tweeted Monday.
The Obamas' visits with Queen Elizabeth and Prince William mark the start of a six-day international trip. The couple will also travel to Germany and Saudi Arabia.