But the Queen and her husband, Prince Philip, were absent from the event, though the palace has said it wasn't for health reasons. People cheered as the attractive Markle walked down the aisle accompanied by Prince Charles and 10 young page boys and bridesmaids, while Harry waited in a dark formal military dress.
Princess Charlotte and Prince George made a memorable appearance on the Buckingham Palace balcony on the afternoon of last month's Trooping the Colour ceremony - part of the Queen's official birthday celebrations.
Nevertheless, only time will tell. (This church has its own significance for the royal family: it was where Charlotte's late grandmother, Princess Diana, was herself baptized.) Also at her christening, Charlotte's five godparents were officially named, including some of her parents' nearest friends and relative.
This was the first time the Duke and Duchess of Cambridge stepped out with all three children.
The Duke and Duchess look so happy - do you have a message of congratulations for the young royal family?
Kate is now in the middle of maternity leave for Prince Louis and hasn't yet returned to full-time royal duties. Having said that she did step out of her comfort zone and wore a more create hat when she attended the Royal Ascot and more recently the centenary of the Royal Air Force.
Louis's siblings Prince George, 4, and Princess Charlotte, 3, also watched the ceremony at The Chapel Royal in St. James's Palace.
William encouraged George and Charlotte to "shake hands" with the Archbishop of Canterbury, before telling her: "Well done".
Lucy Middleton is Kate's cousin and the sister of Adam Middleton, who is one of Princess Charlotte's godparents.
Princes Harry and William were always fond of English author Alan Alexander Milne (known as A.A. Milne professionally), so a book about Pooh Bear sounded like the right way to start the first-editions library.
Following the 40-minute service, The Duke and Duchess of Cambridge gave a private tea at Clarence House.