William undertook a night-time engagement outside Buckingham Palace to play his part in a dual beacon-lighting ceremony with the Queen, who was 22 miles away at Windsor.
Speaking to Mike Bloomberg, sponsor of the Tree of Trees principal beacon, the duke said: “It was a big day today, It was pretty impressive. Did you see the flypast?”
William watched as live footage of his grandmother at Windsor appeared on a large screen, where she was seen illuminating a globe which sent a river of light from the castle to Buckingham Palace.
A run of festooned lights laid on the ground outside the London residence lit up, followed by 3,500 lights on the 69ft tall living Tree of Trees sculpture, made up of 350 saplings.
William, speaking to designer Thomas Heatherwick, said of the creation: “It looks amazing.”
The living sculpture reflects the planting of more than a million trees as part of the Queen’s Green Canopy (QGC) initiative to mark 70 years of her reign.
Heatherwick said afterwards that the Queen was the first to notice the sculpture was as tall as three giraffes.
“We talked about how his grandmother was really responsible for the initiative and how she had described it as being three giraffes high,” Mr Heatherwick said.
“It’s 70ft high and she said, ‘That’s three giraffes’. When it was two-thirds complete, she saw it and said, ‘That’s two giraffes’.”
William chatted with singer-songwriter Gregory Porter, who performed a specially composed song, A Life Lived With Grace, alongside the London Community Gospel Choir.
The duke told the US musician: “Your voice gets even more impressive every time I see you.
“It’s even more moving. It’s brilliant.”
William joked about where the sculpture should end up, saying: “Have you got any ideas where it should go next?”
Porter replied: “Hyde Park. One more tree,” with William laughing and agreeing “One more tree”.
The duke wished Porter well, telling him: “Look after those vocal cords.”
After the Jubilee weekend, the saplings in their pots will be donated to community groups and individuals around the country.
As part of the ceremony, photos of the Queen from each of the decades of her reign were projected on to the front of the palace.
They included one of her on horseback as she took the salute at Trooping the Colour in the 1980s.