Walker described Kate as an “amazing musician” and said the pair got together for top-secret rehearsals in a recording studio ahead of the concert, which was broadcast on Christmas Eve.
Kate hosted the event, Royal Carols: Together At Christmas, as a thank you to the people who have supported their communities during the pandemic.
British singer-songwriter Walker, known for his hit Leave A Light On, said he was approached by the duchess to perform at the carol service after meeting at a charity event – and they ended up jamming together and performing his previously unheard Christmas song, For Those Who Can’t Be Here.
Walker said: “I didn’t even know she played piano.
“It was very secret, very secret – even the studio didn’t know what was going on. We were sitting on opposite sides of the room for Covid, rehearsing.
“So we got together, we rehearsed the song like nine times and by the end of it she’d absolutely nailed it, and then she went away for a couple of days and practised it, and then we finally got to do the recording of it.
“And I was really impressed because it’s one thing playing along with me in a studio, just the two of us, but then to jump straight in to playing with a live string quartet and a pianist and two backing singers, all of which she’d never met before, and then doing live takes in front of the camera – that’s a whole other jump from jamming.
“She absolutely nailed it and I was so surprised at how great she was at keeping time, because she had to start the song off and lead it.
“Amazing musician and what an amazing day, it was crazy.”
A clip on the Kensington Palace Twitter feed teased Kate’s involvement on the piano.
The duchess gained her grade three piano and grade five theory.
Walker said Kate told him “she loved the lyrics”, but did not mention specifically anything to do with the Duke of Edinburgh, her husband’s grandfather, who died in April.
He said: “I think just everybody has lost somebody in their lives at some point and Christmas is the time to remember people who aren’t around who you used to have really fond memories of and still do, but you can’t reminisce with them in person.
“So I think that’s why she connected with the lyrics.”
Walker said of the concert: “I think we were both really nervous that it wasn’t going to go quite to plan and one of us would let down the other person or whatever, but she was absolutely fabulous – she smashed it.
“What a talented, kind, warm-hearted, lovely person.
“She’s very chilled, very nice and she was so kind to everybody who was there – personally introduced herself to the whole band.
“It was just lovely. A big moment for me in my career.”
In a recorded introduction to the service, the duchess paid tribute to the “inspirational” people who have served their communities during the “bleak time” of the pandemic.
Kate said: “We wanted to say a huge thank you to all those amazing people out there who have supported their communities.
“We also wanted to recognise those whose struggles perhaps have been less visible too.”
The duchess said the country had been through “such a bleak time” and had faced “many challenges”, including the loss of loved ones, while frontline workers had been under “immense pressure”.
The service was attended by those Kate and William had spent time with during recent engagements, as well as members of the armed forces involved in Operation Pitting to evacuate British nationals and eligible Afghans from Afghanistan, young carers, faith leaders and those who may have been more vulnerable or isolated during the pandemic.