Close family and friends were invited to celebrate the occasion at the Windsor residence, where a cake by the couple’s wedding baker Claire Ptak was also served.
A spokesperson for the couple did not name specific family members who attended the event.
The picture of Lilibet, which shows her smiling and dressed in a pale blue frock with a white bow in her hair, was taken by photographer and friend of the couple Misan Harriman, who attended the picnic.
Meghan and Harry thanked well-wishers, including some who made donations amounting to over £79,000 in Lilibet’s honour to the World Central Kitchen, a not-for-profit organisation partnered with the couple’s Archewell foundation, according to a spokesperson for the Sussexes.
Baby Lilibet
PA
Meghan Markle and Prince Harry
POOL
The organisation has provided support in Uvalde, Texas and Ukraine, the spokesperson said.
Earlier, Meghan and Harry were pictured arriving at a Californian airport, following their trip to the UK.
Along with their children Archie and Lilibet, they flew out of Farnborough Airport at 1.30pm yesterday, an hour before the Jubilee Pageant began in central London.
The couple, who departed the UK in 2020 for a life in the US, made their first family trip to Britain to celebrate the Queen's 70-year reign.
Meghan and Harry were booed upon arriving at St Paul's Cathedral on Friday and refrained from attending the reception at the Guildhall following the Service of Thanksgiving.
The Sussexes kept a low profile across the Jubilee celebrations and were photographed at the Major General's Office, alongside minor royals, for Trooping the Colour.
The Queen said she was "humbled and deeply touched" by celebrations for her Platinum Jubilee, telling the nation she remains "committed to serving you to the best of my ability".
The 96-year-old monarch was absent from a number of Jubilee proceedings, but remained determined to make one final appearance.
In a later message, Her Majesty said: 'When it comes to how to mark 70 years as your Queen, there is no guidebook to follow.
"It really is a first. But I have been humbled and deeply touched that so many people have taken to the streets to celebrate my Platinum Jubilee.
"While I may not have attended every event in person, my heart has been with you all; and I remain committed to serving you to the best of my ability, supported by my family.
"I have been inspired by the kindness, joy and kinship that has been so evident in recent days, and I hope this renewed sense of togetherness will be felt for many years to come.
"I thank you most sincerely for your good wishes and for the part you have all played in these happy celebrations."