Earlier this year, after Meghan Markle and Prince Harry exchanged vows in St. George's Chapel, the duchess changed into a modern Stella McCartney gown for the couple's intimate evening celebration at Frogmore House.
There had been doubts about whether the 97-year-old Philip would be well enough to attend, but he seemed to be in good form during a rare public appearance.
She also shared a snap of actress and model Liv Tyler who was also dressed in her label.
Eugenie, 28, younger daughter of the queen's third child, Prince Andrew, and his ex-wife, Sarah Ferguson, the Duchess of York, tied the knot with Brooksbank, 32, in a traditional ceremony in the castle's 15th Century St George's Chapel.
Eugenie was reportedly introduced to the former Take That singer by her mother the Duchess of York on a Caribbean holiday in 2011.
Stars like Demi Moore, Ayda Field and Robbie Williams and the Delevingne sisters all put on their finest attire for Queen Elizabeth's reception at Windsor and then the later bash at the grand Royal Lodge.
ASSOCIATED PRESS Princess Eugenie walks down the aisle with her father, Prince Andrew.More news: Nigeria To Witness Sluggish Economic Growth In 2019
At her royal wedding, Princess Eugenie wore a dress with a low-cut back that revealed a long scar from an operation on her spine, inspiring others with back scars to share photos and feel more comfortable with their bodies.
The Windsor royal wedding peaked at 3.9 million when it aired on the channel in an extended This Morning special hosted by Eamonn Holmes and Ruth Langsford from 9.25am until 12.30pm, according to overnight ratings.
ITV was the only United Kingdom broadcaster that showed the entire service live from St. George's Chapel.
The silk for the gown came from the British mill Biddle Sawyer Silk, the palace said.
Singing and cheering well-wishers gathered outside in the streets of Windsor in the shadow of the castle, although there were far fewer people than crammed into the town for Harry's wedding.
"I'm a royal superfan, so when her majesty organizes a big event for her granddaughter, I can't stay at home", said Joseph Afrane, 54.