The 92-year-old monarch has a busy week ahead, marked by a visit from US President Donald Trump in Windsor on Friday.
Prince Louis wore a handmade replica of the Royal Christening Robe, made by The Queen's dressmaker, Angela Kelly. At St James's Palace in London, nearly the entire gang (minus the Queen) was there to celebrate his big day-which he reportedly slept through completely.
Following the 40-minute traditional religious service, guests attended a private tea which included slices of christening cake, from a tier taken from the duke and duchess' wedding cake.
Louis was born on April 24 - St George's Day, England's national day - at St Mary's hospital in central London weighing eight pounds and seven ounces.
The sky display was quite a sight, and the adult members of the royal family were out in full force; the two duchesses looked absolutely stunning in their outfits. The little prince wore the same gown his older siblings wore to their christenings and continued the tradition.
The young prince, who has an older brother George, 4, and sister Charlotte, 3, was baptized using water from the River Jordan.
Catherine's sister Pippa, who is expecting her first child, was also at the special occasion with her husband James Matthews, along with their parents Michael and Carole Middleton.
Prince Louis godparents are Lady Laura Meade, Hannah Carter, Lucy Middleton, Nicholas van Cutsem, Harry Aubrey-Fletcher and Guy Pelley. Prince Harry and Meghan Markle were behind them. The Duke of Sussex purchased a first edition of Winnie-The-Pooh, worth close to $11,000.