The newly minted Duke and Duchess of Sussex (AKA Harry and Megan) followed behind the new family of five, along with Louis' proud grandpa, Prince Charles, who walked in with his wife Camilla, Duchess of Cornwall.
Dressed in a boatneck Ralph Lauren dress, she teamed it with a matching belt and a fascinator by Stephen Jones.
The Queen looked well after skipping Prince Louis's Christening on Monday, following a busy week in Scotland last week.
The "Fab Four" were also there, arriving together with the Duke and Duchess of Cambridge in one auto, followed by the Duke and Duchess of Sussex.More news: British Foreign Secretary Boris Johnson resigns
Also attending were Kate's parents, her brother and sister and Louis' six godparents, who are close friends or relatives of William and Kate.
The Archbishop of Canterbury, Justin Welby, who officiated their royal wedding on May 19, also presided over the christening.
While The Duke of Cambridge became a fourth generation RAF member, with active duty as a search and rescue helicopter pilot.More news: Rescuers prepare to extract final five from flooded Thai cave
Meghan looked stunning as she arrived at Westminster Abbey for the start of the celebrations wearing a black dress by Clare Waight Keller for Givenchy which featured a bateau neckline, just like her wedding dress.
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.
After the service, the monarch, accompanied by Prince Charles, will present a new Queen's Colour to the RAF on the forecourt of Buckingham Palace and make a short speech.
The Royal Family all joined the Queen for the ceremony on the forecourt of the Palace.More news: 3rd mission underway at Thai cave, aims to rescue all left