The photograph was taken on the Windsor Estate earlier this year by Millie Pilkington.
This year marks the first time William will mark the occasion with his father as monarch.
It comes after King Charles marked his first Trooping the Colour as sovereign with a Buckingham Palace balcony appearance.
Charles and the Queen acknowledged with a wave the thousands of well-wishers in The Mall who sang the National Anthem and gave three cheers after the King’s official birthday was celebrated with a display of pomp and military pageantry.
The King and Camilla were surrounded by the family’s core working royals including the Prince and Princess of Wales and their children.
And when he travelled in a carriage with his siblings and mother, Louis held his nose to possibly ward off the smell of horse manure from the many military mounts.