Kate was seen deep in conversation with Camilla during the key diplomatic engagement.
The princess, who wore her hair down, was pictured smiling at the Queen Consort, who was holding a drink with a slice of lemon in it, as the pair chatted.
At one stage, Kate, who was wearing a long three-strand pearl necklace, placed her hand on the arm of Saint Lucia governor-general Cyril Errol Melchiades Charles as they spoke.
The princess also stood next to the King as he conversed with the dignitaries, with William also mingling in the busy room.
In addition to the UK, the King is head of state for 14 Commonwealth realms.
These are Antigua and Barbuda, Australia, The Bahamas, Belize, Canada, Grenada, Jamaica, New Zealand, Papua New Guinea, Saint Kitts and Nevis, Saint Lucia, Saint Vincent and the Grenadines, Solomon Islands, and Tuvalu.
But the monarchy’s future role in some of the countries appears less certain with the start of a new reign.
The prime minister of Antigua and Barbuda, Gaston Browne, said after the Queen’s death that he would call for a referendum on the country becoming a republic within three years.
New Zealand’s prime minister Jacinda Ardern has said her government will not pursue becoming a republic following the death of the Queen.
Australian Prime Minister Anthony Albanese has said “now is not a time” to discuss the country becoming a republic, but he has not ruled out a future referendum on the issue.
Jamaica is expected to follow Barbados, which became a republic in November 2021, although it remains within the Commonwealth.
Jamaica Prime Minister Andrew Holness told William and Kate during their tour in March his country was “moving on” and intended to “fulfil our true ambitions and destiny as an independent, developed, prosperous country”.