The monarch was due to return to London on Monday before a busy schedule of events including a meeting with U.S. President Donald Trump on Friday at her Windsor Castle home.
Prince Louis was sleeping as he arrived at the ceremony, held by his mother.
It will be the first time the new family of five will be seen together in the public.
In addition to the royal family, members of Kate's family - her parents, Michael and Carole, brother, James, and sister, Pippa - are also attending the christening. She teamed it with a hairband-style hat by Jane Taylor, which swept her glossy chestnut hair back into her signature updo.
George also served as a page boy for his aunt Pippa Middleton's wedding in 2017. Lucy Middleton is Kate's cousin and the sister of Adam Middleton, who is one of Princess Charlotte's godparents. They were joined by Prince Charles and the Duchess of Cornwall.
Prince Louis's christening gown is the same one that was used by his siblings. It was hand-made in 2008 by Angela Kelly, the Dressmaker to the Queen.
Welby will use the Lily Font, commissioned by queen Victoria for the baptism of her first child in 1841.
Prince Louis is the fifth in line to the British throne.
Welcome (officially) to the world, little Prince Louis.
Hours before the ceremony, Prince Louis' godparents were revealed in an official statement from the palace.
Prince William could then be heard telling Charlotte: "Well done". He has been banned from driving three times, including twice for drink-driving and once for speeding. Picture: AAPNicholas van Cutsem is another godparent.
(L-R bottom row) The royals - Prince William of Cambridge, Prince Charles of Wales, Camilla of Wales, Kate Middleton of Cambridge, Prince Andrew of York, Princess Beatrice of York and Princess Eugenie of York - attended the wedding in full force in their fashionable best. She is the daughter of the 8th Earl of Romney.
Next is Hannah Carter, an old friend of Kate Middleton's from Marlborough College who is married to Robert Carter.
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