Kate Middleton proudly supported King Charles, Queen Camilla and Prince William at the Garter Day on Monday.
The Princess of Wales looked gorgeous in a dazzling polka dot dress as she proudly supported the royal family.
It's the first annual Garter procession of the King's reign and is one of the most colourful events of the royal calendar reports the Mirror.
It sees royals and others donning grand velvet robes and plumed hats for a procession which runs from the castle to St George's Chapel.
Following the procession, the royals will attend a private service before they return in horse-drawn carriages.
Taking part in the event alongside senior members of the royal family were Princess Anne, Prince Edward and the other Knights and Ladies of the Garter.
Prince Andrew was nowhere to be seen despite wearing his Order of the Garter velvet robe and plumed hat when he attended the Coronation in May.
The shamed royal was understood to have joined King Charles and other members of the royal family earlier on Monday for the investiture in the castle's Garter throne room and at the traditional lunch that takes place afterwards in the Waterloo Chamber, both of which took place behind closed doors.
At St George's Chapel, Kate, as well as the Duchess of Edinburgh, proudly supported their family from the sidelines as they are not members of the order.
They arrived by car prior to the procession to watch it arrive at St George's Chapel, with Kate beaming with pride as she spotted her husband William.
Following the ceremony in the chapel, Kate and Sophie joined the other royals in a carriage procession back to Windsor Castle.
The Most Noble Order of the Garter was established by King Edward III nearly 700 years ago.
It is the oldest British order of chivalry and the world's oldest national order of knighthood still in existence.
Members of the Order include the King, several senior royals and other Knights or Ladies, who are chosen by the monarch to recognise their work.
The Order is limited to 24 Knights or Ladies Companion, in addition to the King, Prince of Wales and other several "supernumerary members", including some foreign monarchs.
Recipients of the honour are chosen due to their contribution to public office, to national life or served the sovereign personally.
The appointment of Knights and Ladies of the Garter is in the King's gift and is made without consulting ministers.
During Monday's service, the Baroness Ashton of Upholland was installed as Lady Companion of of the Most Noble Order of the Garter, alongside the Lord Patten of Barnes installed as a Knight Companion. Their appointments were announced earlier this year, on April 23.
Baroness Ashton is a former Labour government minister and EU diplomat who was appointed a Dame Grand Cross in the 2015 New Year Honours for services to international diplomacy.
Lord Patten, meanwhile, is a former Tory MP who served as the final Governor of Hong Kong from 1992 until 1997. His parliamentary career spanned over two decades, and it included serving as the environment secretary, Chancellor of the Duchy as well as chairman of the Tory party.
He was also chairman of the BBC Trust between 2011 and 2014 and was appointed as a companion of honour in 1998.
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