Around 100 heads of state and 20 royal families from around the world are expected to attend Charles' coronation Saturday at Westminster Abbey.
Charles was also photographed speaking with Zelenska during the reception, which the first lady attended with Denys Shmyhal, the prime minister of Ukraine.
Biden was spotted at a different point in the reception having a second conversation with Kate alongside her granddaughter, Finnegan Biden, who accompanied the first lady to the U.K.
Earlier in the day, Kate and Charles were joined by Prince William as they surprised fans waiting outside ahead of Saturday's festivities.
The trio were seen shaking hands with fans and taking pictures along the same procession route that Charles and his wife Camilla, the Queen Consort, will take from Buckingham Palace to Westminster Abbey for the coronation service.