She styled the belted boat-neck pencil skirt dress with a matching wide-brim hat by British milliner Stephen Jones.
A day after gathering for the youngest royal heir's special church service, Prince William, Kate Middleton, Prince Harry, and Meghan Markle helped celebrate the 100th anniversary of the Royal Air Force.
"The bun is quickly becoming the Duchess's signature look", hairstylist Jason Collier told the Daily Mail, "so it's no surprise to see her sporting this style at today's ceremony".
Prince William and Catherine have chosen three godmothers and three godfathers who are all close friends and family members, not including Prince Harry and Meghan, to be little Louis' godparents.
The Queen looked well after skipping Prince Louis's Christening on Monday, following a busy week in Scotland last week. A few days ago, the Duchess of Sussex ditched her pastel outfits and wore a bright yellow ensemble while attending an event with Prince Harry.
Pippa and her big sister have a similar taste in fashion, as Pippa wore a stunning baby blue Alessandra Rich dress which looked similar to the Alexander McQueen frock that the Duchess wore at her wedding in 2017.
Queen Elizabeth and senior members of her family were attending celebrations to mark the 100th anniversary of Britain's Royal Air Force on Tuesday, ahead of a flypast of nearly 100 aircraft over Buckingham Palace in central London.
Although the Duchess is not a cadet they seem to think she was deserving of the brooch.
Once inside the abbey, all the royals were greeted by the Archbishop of Canterbury, Justin Welby, who shook their hands before ushering them to their seats. But it was largely hidden behind Prince Louis' christening gown.