The US-based couple, who are not working royals, travelled to England with their two children, Archie and Lilibet, for the Queen's Platinum Jubilee celebrations, marking 70 years on the British throne.
Close friends and family were invited for a "casual, intimate backyard picnic" on Saturday at Frogmore Cottage, a spokesperson for the couple told People.
The Duke and Duchess' house is near Windsor Castle, Queen Elizabeth's primary residence.
In the photo, Lili sits in the yard of her family's home wearing a blue dress and a white lace bow in her red hair.
Misan Harriman, a close family friend who attended the party, took the "candid, spontaneous photograph that the Duke and Duchess are pleased to share today," the spokesperson added.
Baker Claire Ptak from Violet Bakery in East London created a special birthday cake for Lilibet. Ptak also made Harry and Meghan's wedding cake in 2018.
The trip was Lilibet's first visit to the United Kingdom, as she was born in California. Archie was born in England in May 2019 but hadn't been back to visit since he was six months old.
Lilibet also got to meet her great-grandmother, the Queen, for the first time during the trip. She is named after the monarch, as Lilibet was the Queen's childhood nickname.
The children didn't attend any of the Jubilee events, but Meghan and Harry appeared at Trooping the Colour and a thanksgiving service in honour of the Queen at St Paul's Cathedral.
They did not attend the Platinum Party at the Palace on Saturday night or the Platinum Jubilee Pageant outside Buckingham Palace.
Several of the royals marked Lilibet's birthday on social media, with the Queen, Prince William and Kate Middleton and Prince Charles and Camilla sharing their birthday tributes.
The Sussexes moved to the United States to lead a more independent life in 2020, and have since delivered some stinging attacks on Buckingham Palace and the royal family, though not the Queen.
They returned to their home in California on Sunday.
