The awards in the Royal Victorian Order are given personally by the queen for services to the Sovereign and Catherine has been given the highest ranking possible.
The queen is able to give out Knight or Dame Grand Cross (GCVO), Knight or Dame Commander (KCVO or DCVO), Commander (CVO), Lieutenant (LVO) and Member (MVO) and has previously bestowed them on her husband Prince Philip, sons Princes Andrew and Edward, her daughter Princess Anne, and Camilla, Duchess of Cornwall and Sophie, Countess of Wessex, the wives of her sons Princes Charles and Edward.
Catherine and William's official Twitter account marked their anniversary with a selection of pictures from their wedding day and thanked well-wishers for their kind messages.
A post read: "8 years ago today, thank you for your lovely messages on The Duke and Duchess of Cambridge's wedding anniversary!"
The 37-year-old duchess - who has children Prince George, five, Princess Charlotte, three, and Prince Louis, 12 months, with her spouse - has previously praised the queen for her "thoughtfulness" and admitted she thinks the first ever Christmas gift she gave the monarch, a jar of homemade chutney, went a "long way" in establishing their relationship.
She previously said: "[The Queen] really cares.
"I can remember being at Sandringham, for the first time, at Christmas. And I was worried what to give the Queen as her Christmas present.
"I was thinking, 'Gosh, what should I give her?' I thought back to what I would give my own grandparents. And I thought, 'I'll make her something.' Which could have gone horribly wrong.
"But I decided to make my granny's recipe of chutney. I was slightly worried about it, but I noticed the next day that it was on the table.
"I think such a simple gesture went such a long way for me and I've noticed since she's done that on lots of occasions and I think it just shows her thoughtfulness, really, and her care in looking after everybody."