The royals were greeted at the abbey doors by the Dean and Chapter of Westminster, before taking their seats.
It has also been noted that Kate's decision to wear white or cream at the christening of Prince George, Princess Charlotte, and now Prince Louis is due to the colours perfectly complementing the Royal christening gown, worn by all three children. The stiletto came in a nude shade and paired beautifully with the christening robe - a replica designed by Queen Elizabeth's dressmaker, Angela Kelly - draped over the 11-week-old baby in her arms.
After the royal appearance, the family watched the flypast from a balcony at Buckingham Palace with great ceremony.
The Duke and Duchess of Sussex arrived at the Abbey together with the Duke and Duchess of Cambridge, with the Princes in their uniforms and Kate Middleton again sporting Alexander McQueen. Kate Middleton went with an icy blue dress by her go-to designer, Alexander McQueen, and a hat by Sean Barrett. Later, Meghan and Harry will head to Dublin to begin their two-day whirlwind trip of Ireland.
Before entering the chapel, Kate Middleton stopped by the Archbishop of Canterbury, Justin Welby, who said with a smile that Louis was "very calm and serene".
Kate last wore the broach - once the highest award any woman of the rank could achieve - during a service for the 75th anniversary of the RAF Air Cadets in 2016.
DOMINIC LIPINSKI/PA WIRE/PA IMAGES The family at Prince Louis' christening on July 9.
The family has a long-standing commitment to the Royal Air Force with four generations of the family having served or trained with the United Kingdom's aerial warfare force, and many holding honorary ranks and patronages with RAF charities and associations.