The Prince was given the title which had previously been bestowed upon his father - the new monarch - for more than 64 years following the death of his grandmother, the Queen, on Thursday.
The new King hosted a reception with Realm High Commissioners and their spouses in the Bow Room at Buckingham Palace, pictured smiling as he greeted officials from across the world.
The 75-year-old royal was elegant in a black dress, with matching heels, teamed with a golden necklace and brooch as she attended the event at the London residence.