The royals were greeted at the abbey doors by the Dean and Chapter of Westminster, before taking their seats.
After the royal appearance, the family watched the flypast from a balcony at Buckingham Palace with great ceremony. It was the first time that Prince William and Middleton were spotted with their three children. Earlier, the British saw only his birthday, April 23. They're the COOL Duke and Duchess of Sussex-who continue to defy royal protocol in understated, endearing ways.
The Duchess of Sussex looked stunning in a demure, belted black dress from Dior with her favored boatneck neckline and a wild, Stephen Jones fascinator that she makes look chic. She paired the look with nude heels and a clutch.
The dress appeared to be the exact same style to the lemon one she wore to Prince Harry and Meghan Markle's May wedding, which is also identical to the cream one the Duchess wore for Princess Charlotte's Christening in 2015.
Missing was the queen's husband, Prince Philip, 97, who has retired from public life.
Meghan and Harry will head straight from the service to Dublin for a two-day tour.
Kate has attended a handful of events tied in with the RAF Cadets.
Meghan, Duchess of Sussex joined Prince Harry, Duke of Sussex, Queen Elizabeth, the Duke and Duchess of Cambridge, Prince Charles and Duchess Camilla at the RAF Centenary where they all attended a Royal Air Force Westminster Abbey service.
After a busy week in Scotland last week, the queen faced another busy week ahead in London, including a meeting with visiting President Donald Trump and first lady Melania Trump on Friday.
The Duchess of Cambridge is thought to have given a special nod to her late mother-in-law on Monday (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.