The food was plant-based, the flowers grown locally and the carpet, as is now tradition, was green.
Guests invited to the second annual Earthshot Awards were whisked to the venue in electric taxis and encouraged to recycle pre-loved outfits.
The Princess of Wales, no stranger to using clothes to send a message, led by example, sending the crowds into a frenzy as she arrived at the MGM Music Hall, Boston, in an emerald green, off-the-shoulder Solace London gown, rented for £74 from online platform HURR.
She opted to wear an emerald choker that had belonged to her late mother-in-law, Diana, Princess of Wales, handed down, via Queen Elizabeth II, from Queen Mary.
The Prince and Princess received a rapturous welcome, pausing for photographs outside before taking part in the star-studded show, hosted by Clara Amfo and Daniel Dae Kim.
Prince William, wearing a navy velvet dinner jacket, opened the ceremony with a pre-recorded piece to camera in which he spoke of the Earth’s “splendour”, which he said gave curiosity, joy and life.
“For too long, many of us have taken for granted everything our planet gives us,” he added.
“We are polluting our atmosphere and destroying many unique habitats.
“This year, we've seen the influence of climate change like never before, with record high temperatures and extreme weather events causing devastation across every continent, affecting people's health and livelihoods.”
The awards, masterminded by William, recognise efforts and inventions to solve key environmental problems, from air pollution to climate change.
Five budding winners each won £1 million to scale up their projects.
“A million dollars is significant,” Justin Winters, executive director of climate philanthropy organisation One Earth told the Boston Globe.
“For a lot of these nonprofits and companies, it’s a game-changing investment. But I would say in this case, the level of attention and partnerships that are brought to the table — to the finalists and the award winners — are really significant.”
In order to reduce the carbon footprint, all finalists were filmed on home turf, joining the ceremony via a live video feed.
The Princess was among those who presented an award, warning that air pollution “poses a threat that knows no borders.”
Guests were treated to performances by Ellie Goulding, Annie Lennox, and Billie Eilish.
Among the winners was Notpla, a London-based start-up that makes biodegradable packaging from seaweed and plants.
The brainchild of friends Pierre Paslier and Rodrigo Garcia Gonzalez, it was used during the 2019 London Marathon when 36,000 Notpla-made edible bubbles, filled with Lucozade, were handed to runners.
This year, Notpla has made more than one million takeaway food boxes for Just EatTakeaway.com.
Mr Paslier said: “When Rodrigo and I started Notpla eight years ago in our student kitchen, we would have never imagined we would be here today.
“No one wants to live in a world full of plastic waste but it’s not too late to act. There’s never been a greater time to use natural solutions to solve the plastic challenge.”
After the show, the Prince and Princess went backstage to thank the celebrities who had taken part.
In a private room, they chatted to performers including Lennox, who had sung Here Comes The Rain Again while a light show on stage depicted a dry patch of earth transforming into flowers in bloom.
Prince William said to her: “The set was amazing. Thank you.”
A passionate campaigner, Lennox spoke about the importance of highlighting the Earth’s problems, telling him: “I’m on your side.”
The couple also chatted to Beyonce proteges Chloe x Halle, who asked the Princess to pose for a picture with them.
Actress Shailene Woodley, who had presented an award, asked if the plan was to do a similar Earthshot show every year.
The Prince said: “Yes, that's the plan. We want to big it up. To have it in a different place every year: China, Indonesia, South America, Australia, New Zealand. We have to work out where to go and when.”
He joked: “That's the hard bit. Everything else is easy.”
For the Prince and Princess, the ceremony wrapped up a busy three-day trip that had threatened to be derailed by a royal race row and the release of a dramatic trailer promoting Harry and Meghan’s forthcoming Netflix documentary series.
However, they ploughed on and refused to allow such distractions to tarnish the visit, which included a 30-minute meeting between the Prince and US President Joe Biden.
The royals flew home on a British Airways flight overnight to be reunited with their three children.