Following the 40-minute service, The Duke and Duchess of Cambridge gave a private tea at Clarence House.
This was the first time the Duke and Duchess of Cambridge stepped out with all three children.
It's also where the coffin of Diana, Princess of Wales laid for a week before her burial.
The little girl even thought to bring the order of service home with her, tucking the large booklet under her arm and you have to admire that level of efficiency, she's clearly taking the matter of her brother's christening very seriously.
Mrs. Robert Carter aka Hannah Gillingham is one of Kate Middleton's best friends from Marlborough College.
Despite still being on maternity leave after giving birth to Prince Louis only 11 weeks ago, it is understood her mother Kate felt it was "important" to attend today's celebrations.
The duchess, who was wearing Alexander McQueen, added, with a smile: "I hope he stays like this".
While Harry also wore a blue suit and light blue tie, Meghan opted for an olive Ralph Lauren dress with a matching headpiece and Manolo Blanik pumps.
While Louis has six godparents, his older brother George has seven, while Charlotte has five.
Harry Aubrey-Fletcher: He went to Ludgrove School and Eton with William and his father is the Lord Lieutenant of Buckinghamshire.