Mother and daughter sat together at the instrument to play a piece they knew well by Scottish composer Erland Cooper.
The performance was for the Together at Christmas concert, held at Westminster Abbey on December 5, and was pre-recorded last week so did not feature at the live event - but was part of the ITV1 screening of the service on Christmas Eve.
For the past five years, Kate has staged her celebration of Christmas, and at the inaugural event, she surprised audiences by accompanying singer-songwriter Tom Walker on piano as he sang his Christmas song, For Those Who Can't Be Here.
For her latest performance, the princess played Cooper's piece Holm Sound using only her left hand, while Charlotte played with just her right in Windsor Castle's Inner Hall.
The princess and her daughter have enjoyed playing the piece together at home, and as they performed, footage was shown of guests arriving for the carol service - with the Prince of Wales and Kate watching as their children tied paper chains bearing their names on a "Connection Tree" outside the abbey.
The princess also narrated the letter that accompanied every order of service, where she reflected on the Christmas period that reminds us "how deeply our lives are woven together".
And despite life feeling "fragmented or uncertain" at times, she wrote the festive season "invites us to remember the power of reaching out to one another".
The pair clearly enjoyed performing together, smiling at each other, and it is understood Kate wanted to include the musical element in the carol service to highlight her belief in the importance of connections - as mentioned in her letter.
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