Kate, 40, stunned in a vintage-inspired pastel blue-green dress, custom made by her private dressmaker.
The custom-made dress featured a high neck and fabric-covered button-down to the waist, where the skirt flowed out in a fit-and-flare style that the duchess has become known for.
It's not the first time Kate has taken the elegant dress for a spin, she previously wore the style to attend Prince Charles' 50th anniversary of his investiture in 2019.
For Wednesday's outing, the duchess accessorised the spring look with an eye-catching hat in a matching pastel shade that featured a larger-than-life flower, and opted for coordinating suede stiletto heels and drop earrings.
Prince William joined his wife for the rainy afternoon, opting for a full morning suit, complete with top hat and tailcoat.
Whilst the occasion was the first Garden Party for Prince William since his last appearance in 2019, Kate attended last week's gathering in a coral ensemble by one of her go-to brands, Emilia Wickstead.
The Duke and Duchess of Cambridge were joined by William's uncle and the Queen’s youngest son, Prince Edward, and his wife, Sophie, Countess of Wessex, who wore a vibrant blue dress by Emilia Wickstead, with a similar silhouette and shade to one that Kate wore by the designer in the Caribbean earlier this year.
The two couples have formed quite the royal 'dream team' in recent weeks, taking a 'divide and conquer' tactic when standing in for the Queen on many occasions including during royal tours and public appearances in the lead up to Her Majesty's Platinum Jubilee.
Also in attendance was Prince William's cousin, Princess Beatrice, who beamed as she greeted guests in a chiffon checked midi dress and geometric-style headband.
The Queen, who turned 96 in April, usually welcomes over 30,000 guests every year to four Garden Parties over the course of the summer, three at Buckingham Palace and one at the Palace of Holyroodhouse in Edinburgh.
Garden Parties are a way for the Queen to speak to a broad range of people and celebrate those who have made a positive impact in their community and have distinguished themselves in public service.
However, this year Her Majesty is being represented by senior members of the Royal Family after withdrawing from certain public engagements amid ongoing 'mobility issues'.
The Queen pulled out of the State Opening of Parliament two weeks ago due to her health, and Prince Charles made the Queen’s speech for the first time.
Princess Anne and Prince Charles each hosted the first two of the annual Garden Parties on behalf of their mother, the Queen, and the third, held last week in the grounds of Buckingham Palace was hosted by the Duchess of Cambridge, the Earl and Countess of Wessex and Princess Alexandra, one of the Queen's first cousins.
However, the Royal Garden Party requires the royal hosts to be on their feet for over an hour as they greet the hundreds of guests gathered on the Buckingham Palace lawn, so is more challenging physically.
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