The Duke and Duchess of Sussex hosted a 'relaxed' garden party for Lilibet with birthday cake, balloons, party games and picnic snacks.
Like her brother Archie, Lilibet has red locks and was seen wearing a blue dress with a white bow in her hair.
The photo was taken by a close family friend and guest at the party, British photographer Misan Harriman, who also shared a number of other snaps from the party.
"It was a lovely do and had everything you’d expect from a child’s birthday party," a source told The Sun.
"But there was no formal entertainment. The idea was for it to be very relaxed and casual, with people free to pop in and out as they wished.
"It was a nice opportunity for the other royal children to meet Lilibet for the first time."
Baker Claire Ptak from East London's Violet Bakery, who made Harry and Meghan's wedding cake, baked Lilibet's cake for the occasion.
The couple's spokesperson said they "remain incredibly touched by the countless birthday wishes for their daughter, Lilibet Diana Mountbatten-Windsor".
They were "amazed to learn that people around the world made donations" amounting to over $100K to the World Central Kitchen (WCK) in her honour", the spokesperson added, saying the pair "extended their gratitude to all who have shown support for the organization, which is especially near to their hearts."
It comes after Page Six reported that Harry and Meghan invited Prince William, Kate Middleton and their three children to the party, but they did not attend and instead travelled to Wales for the Platinum Jubilee.
The Sussexes trip to the UK gave Lilibet the chance to meet her great-grandmother the Queen. Her Majesty's childhood nickname was the inspiration for Lilibet's name.
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