Jessie, who cared for Princes William and Harry as children, was spotted leaving the Chapel Royal at St James's Palace after the ceremony.
The expert nanny happily chatted away to Kate's father, Michael Middleton, and was most likely on hand to help the first-time parents during the baptism.
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While William and Kate were adamant they would care for Prince George themselves in those precious first few weeks, the royal couple turned to Jessie as a familiar face.
Jessie is thought to have first joined the new family in September when they were still based in Anglesey, and when she was spotted helping Kate with Prince George at a McDonalds car park, en route to Kensington Palace.
Jessie was an obvious choice for the position of royal nanny, with William and Harry showing great affection to their former carer up to this day. She was a guest at both William's 21st birthday party and his wedding to Kate in 2011.
Notable guests at Wednesday's intimate family affair included Prince George's great-grandmother the Queen and her husband Prince Philip, as well as the baby's grandfather Prince Charles, his wife the Duchess of Cornwall, and baby George's uncle, Prince Harry.
On Kate's side of the family, the Duchess' parents Carole and Michael Middleton attended, as did her siblings James and Pippa.
The only other guests at the christening, according to a statement revealed by Kensington Palace, were "Prince George's Godparents and their spouses."
These include Prince William's cousin Zara Tindall, and Julia Samuel, who was a close friend of William's mother Princess Diana.