The ninth-in-line to the throne chose the British designer to create the gown she will walk down the aisle in.
Demi, meanwhile, was in good company on Princess Eugenie's big day.
The dress was one of the most copied of her time, and afterward donated to the Metropolian Museum in NY.
Eugenie's sister, Princess Beatrice, served as maid of honour, while Jack's brother Thomas was best man.
There were rumours that the day would be awkward for the mother of the bride due to her restrained relationship with the royals, and particularly Prince Philip.
The princess specifically requested the low back on the gown, which showcased her scar from a childhood battle with scoliosis. She is now ninth-in-line to the throne and works at the art gallery Hauser & Wirth. The pair met on a skiing vacation in Switzerland in seven years ago, and Brooksbank proposed in January when the pair were in Nicaragua. They married in the same venue used in May by Harry and Meghan.
Theodora Williams, 6, who's the daughter of pop star Robbie Williams and wife Ayda Field also joined the group while Louis de Givenchy, 6, son of Zoe and Olivier De Givenchy joined Prince George as a page boy.
Fergie has previously gushed on social media about the wedding of her daughter and Jack, saying: "We really believe in that, you can see that the York family unity - it's all about seeing the light in life".
The bride's parents left the chapel together smiling as the newlyweds embarked on a horse-drawn carriage ride through parts of Windsor.
The Queen will then host a reception for the couple and their guests at the castle.
"One particular passage in which Jay Gatsby is described reminded her immediately of Jack", Conner said. Charity guests and 1,200 members of the public have been invited into the grounds for the occasion.
There were also crowds of well-wishers on the streets outside the castle.