Welcomed on the dais in the castle's Quadrangle, the first couple was greeted by a Guard of Honor to give them a royal salute.
The Queen and the Bidens had tea at the meeting, their first since Biden assumed the presidency.
According to People magazine, President Biden is the 13th U.S. president the Queen has met in her historic reign, and their warm face-to-face was her first major appearance since the death of her husband Prince Philip at age 99 two months ago.
Vice President Kamala Harris was making history of her own back in the U.S. — joined by Second Gentleman Doug Emhoff, she became the first sitting U.S. VP to march in an LGBTQ Pride event when the couple appeared at Capital Pride in D.C. Saturday.
Wearing a "LOVE IS LOVE" tee under a salmon jacket, Vice President Harris used the appearance to push for passage of the Equality Act and to speak up for transgender rights.
"There is so much more work to do," she said, "and I know we are committed."