The three-year-old was walking hand-in-hand with her dad Prince William after the service at the Royal Chapel at St James's Palace told waiting photographers: "You're not coming".
Guests included newlyweds the Duke and Duchess of Sussex.
But the Queen and Duke of Edinburgh did not attend, with the decision made "some time ago" and not on health grounds, Buckingham Palace said.
Press Association A sleeping prince!
BBC reports that Middleton was overheard saying Louis was "very relaxed and peaceful", and joked to the archbishop that she hopes "he stays like this". Kensington Palace tweeted a video of guests arriving at the ceremony.
Kate wore a V-neck dress with puff sleeves that fashion trackers at What Kate Wore identified as being from Alexander McQueen, her go-to fashion house.
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The Sturgess family added: "Our thoughts and prayers also go out to Charlie and his family and we wish Charlie a speedy recovery". Ms Sturgess leaves behind two sons, aged 23 and 19 and a ten-year-old daughter.
Kate's sister Pippa is expecting her first child.
Prince William and the former Kate Middleton were joined at the christening by both of their families. William was an usher in their wedding and James Meade is godfather to Princess Charlotte.
Van Cutsem's father was one of Prince Charles's closest friends.
She was dressed in another Peter Pan-collared dress, similar to the blue frock she wore at the christening on Monday.
Prince Louis's christening gown is the same one that was used by his siblings.
Queen Elizabeth, 92, has also begun to scale back her duties because of her advancing age, handing over some of her responsibilities to her heir, Prince Charles.