It marks a rare public appearance for Kate during her six months maternity leave, who has only been pictured on official royal business three times since giving birth to her third child.
Louis' godparents are a mix of close friends and family members: Nicholas van Cutsem, Guy Pelly, Harry Aubrey-Fletcher, The Lady Laura Meade and Robert Carter are all friends of the Duke and Duchess' and and Lucy Middleton is Kate's cousin. That gown was worn for all Royal Christenings, including The Queen, her children, and seven of her eight grandchildren.
Three-year-old Charlotte grabbed the headlines when she walked hand-in-hand with her dad past all the press that were waiting outside the chapel.
They are all friends or family of the Cambridges, who also picked friends and confidantes as Prince George's and Princess Charlotte's godparents.
The Queen and Duke of Edinburgh will not be on hand for the event.
Princess Charlotte had a sassy message for photographers as she left her brother's christening on Monday: "You're not coming". Duchess of Cambridge Kate Middleton wore a white structured Alexander McQueen dress with a Jane Taylor headpiece.
Boris Johnson leaves Carlton Gardens after his resignation
The British government says Foreign Secretary Boris Johnson has resigned, amid a widening split in the Cabinet over Brexit . Prime Minister Theresa May's office said May accepted the resignation on Monday and will announce a replacement soon.