Prince William personally invited his family members to join him at the afternoon soiree, as his wife, the Princess of Wales, continues her recovery from cancer treatment.
The rain didn't put a dampener on proceedings as William and his cousins mingled with guests, including a number from William and Kate's patronages, affiliations and passion projects including; Homewards, RAF Valley, Swim England and tenants across Duchy of Cornwall sites.
The future king, 41, joked that it was "perfect weather for swimming" as he met with Douglas Whitlam, President of Swim England, and later revealed that his son Prince George, ten, was a "potential pilot in the making" as spoke with Reverend Squadron Leader Chrissie Lacey, who produced a bag of her freshly baked cookies for self-confessed biscuit lover William.
William also left with a children's book he pledged to read to Prince Louis at bedtime from budding author Rowan Aderyn, part of the Prince's Homewards team in Newport, Gwent.
In recent years, many royal fans have often questioned why the Prince of Wales's cousins do not carry out duties, particularly at a time when there is pressure on a smaller contingent of working royals.
WATCH: Prince William joined by royal cousins at palace garden party
However, Beatrice, Eugenie, Zara and Peter have all carved out their own careers. Beatrice works for US-based tech company, Afiniti, while Eugenie is a director at art gallery Hauser & Wirth in London.
ROYAL GARDEN PARTY STYLE
Zara is a competing equestrian while older brother Peter is head of partner acquisition for the sports and entertainment rights agency CSM Sport & Entertainment.
The late Queen's grandchildren do attend large-scale royal events and each support their own charities and causes.
See the best photos from the Buckingham Palace garden party below...
On Tuesday, it was the turn of the Prince of Wales to host one of the sovereign's annual garden parties, which each see around 8,000 guests invited to enjoy tea and cake in the beautiful gardens of Buckingham Palace. There was time for a quick top hat adjustment before Prince William joined the proceedings.
Zara looked absolutely stunning in a pale pink A-line dress with fuchsia buttons - a Laura Green repeat from Royal Ascot 2022. The mum-of-three teamed her frock with a rose pink hat and matching suede court shoes.
Meanwhile, former rugby star Mike donned a morning suit, matching his pink tie to his wife's ensemble.
It marked the first time that Zara and Mike have joined a royal party at the palace.
Zara's paired her candy-pink hat with a pearly manicure
A closer look at Zara's beauty look with an elegant manicure. The equestrian opted for soft smoky eye makeup and an updo, with loose tendrils framing her face.
Sisters Beatrice and Eugenie wowed with their garden party looks, coordinating with cousin Zara in pink and white hues.
Beatrice, 35, wore the Christina Crimson Dress by go-to label, Beulah London, featuring a high neck and delicate gold button detailing. She teamed the vibrant frock with strappy nude heels and a signature headband.
Meanwhile, Eugenie, 34, looked stunning in a white satin midi dress.
William was also joined by the Duke and Duchess of Gloucester
The Prince of Wales was also joined at the garden party by the late Queen Elizabeth's cousin, Prince Richard, Duke of Gloucester and his wife, Birgitte, Duchess of Gloucester.
Every summer, the monarch traditionally hosts three Garden Parties at Buckingham Palace, and one at the Palace of Holyroodhouse in Scotland. Historically, Garden Parties took the place of presentation parties attended by debutantes, and have evolved into a way of recognising and rewarding public service.
Peter supported his cousin, William, at the garden party
Peter shares a close bond with William and Kate, and was seen cheering on his cousin at a charity polo match in Windsor last summer. Peter split with his girlfriend Lindsay Steven earlier this year and is now in a relationship with Harriet Sperling. Earlier this month, the new lovebirds were seen holding hands at the Badminton Horse Trials.
"We really enjoy going to watch Zara compete – and Badminton [Horse Trials] was a lovely weekend," he said. "I think they all really got a sense of what she does this year and it is always great to be able to support her as a family."
Mike and Zara are parents to Mia, ten, Lena, who turns six on 18 June, and three-year-old Lucas. And the family will be supporting Zara throughout the next few months, planning their summer around her competitions.
"[We will be] hopefully getting some time away but Zara's season is in full swing so will just be a juggle around that and spending time watching her as well." He continued: "It's very hard to sum up a typical day – with horses, dogs, kids and work I think I’d say no day is the same but that keeps things really fun and interesting!"
And if you've ever wondered how royal ladies avoiding getting the heels of their shoes stuck in the mud on rainy days, Zara was wearing protective stoppers on her footwear. Genius!
Princess Eugenie opted for a bridal-esque satin tea dress by Ulla Johnson with Valentino studded heels and a pink headband with bow detailing by Emily-London X Alice Archer.
Her appearance at the garden party comes a day after she attended an ultra glamorous lunch with sister Beatrice at the Audley pub in Mayfair, before Beatrice hosted a dinner to celebrate designer Mark Cross' fabulous collaboration with fragrance company AERIN.
William looked thrilled as he was handed a colourful piece of artwork from one talented guest at the party.
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