Coronavirus restrictions meant the service did not take place this year, but it is something most royals have joined the Queen for each year.
Kate attended her first Christmas Day service in 2011 and most recently in Christmas 2019.
Over the years, Judi James suggested Kate’s confidence has grown and she has played an important part in the family.
Speaking exclusively to Express.co.uk, she suggested the Duchess of Cambridge has managed to cut through any royal tensions.
The expert explained this was most notable in 2018 when she appeared with her brother-in-law Prince Harry, 36, and Meghan Markle, 39.
She added: “Kate appears to have been the broker for peace and later the mender of a very fractured royal Firm.
“In 2018, it was the most difficult royal Christmas walk to church for Kate, who had clearly been tasked to single-handedly lay to rest all the swirling rumours about a fallout between herself and her pregnant sister-in-law Meghan.
“Kate did a professional job of walking next to Meghan and engaging her in a very cheerful looking conversation.
In 2019, Kate and Prince William brought their three young children to the church service for the first time.
At that time, the Duchess of Cambridge seemed to secure herself as a leader within the family, Judi added.
She said: “Last year was Kate’s moment as the new, deserving star and potential leader of the royal firm as she stands oozing confidence at the front of the royal group, taking her moment in front of the cameras with Charlotte with a very calm smile of pride.
“Kate looks strong and protective here, with her raised-brow eye-smile creating confident connections with the crowds and the cameras and her wide, congruent mouth-smile looking relaxed and authentic.”