The Princess of Wales looked dazzling in polka dots as she proudly supported King Charles, Queen Camilla and Prince William at Garter Day today.
It is the first annual Garter procession of Charles' reign and is one of the most colourful events of the royal calendar.
It sees royals and others donning the grand velvet robes and plumed hats for a procession from the castle to St George's Chapel for a private service before they return in horse-drawn carriages.
Taking part in the procession alongside William, the King and Queen today were Princess Anne, Prince Edward and the other Knights and Ladies of the Garter.
Prince Andrew was not among those in the parade and his absence comes after he attended the Coronation last month, where he wore his Order of the Garter velvet robe and plumed hat.
Like last year, the Duke of York was understood to have joined the King and the rest of the family earlier today for the investiture in the castle's Garter throne room and the traditional lunch that takes place afterwards in the Waterloo Chamber, both of which took place behind closed doors.
At St George's Chapel today, Kate, as well as the Duchess of Edinburgh, watched the procession from the sidelines as they are not members of the order, but attend each year to lend their support.
For the occasion, Kate wore a polka dot dress by Alessandra Rich and a black and white hat by Philip Treacy. Sophie opted for a bright floral dress by Emilia Wickstead with a bright pink headpiece by Jane Taylor Millinery.
At one point they had to hold on to their hats as breezy conditions whipped around Windsor.
They arrived by car ahead of the procession to watch it arrive at St George's Chapel and were seen engaging in chat before Kate was seen beaming as she spotted husband William in the procession.
After the ceremony in the chapel they then joined the other royals in a carriage procession back to Windsor Castle.
The Most Noble Order of the Garter is the oldest British order of chivalry and the world's oldest national order of knighthood still in existence. It was established by King Edward III nearly 700 years ago.
Members of the Order include the King, several senior royals and other Knights or Ladies, who are personally chosen by the monarch for recognition of their work. They include former Prime Ministers Tony Blair and John Major as well as Baroness Amos.
It is limited to 24 Knights or Ladies Companion, in addition to the King and the Prince of Wales and several "supernumerary members", including some foreign monarchs such as the King of Spain and the King of the Netherlands.
Recipients of the honour are chosen because they have held public office, contributed 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.
The investing of new companions with the Order's Insignia is a key feature of the annual ceremony, and is done by the monarch.
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.
