The Queen made G7 leaders laugh as she asked "are you supposed to be looking as if you're enjoying yourself?"
Known for her wit, used to put nervous guests at ease, the Queen's quip came as presidents and prime ministers smiled for the camera during a family photo after an evening reception.
The leaders found her quip amusing and Prime Minister Boris Johnson said emphatically "yes" in answer to her question.
Following a day of political talks, the "soft diplomacy" of the monarchy was evident when three generations of the royal family gathered to welcome the leaders to Cornwall.
The Queen hosted an open-air reception at the Eden Project in Cornwall and was joined by the Prince of Wales, the Duchess of Cornwall and the Duke and Duchess of Cambridge.
William and Kate are taking part in their first G7 events - another milestone in their progression as senior royals.
The leaders, from German Chancellor Angela Merkel to Canada's Prime Minister Justin Trudeau and US President Joe Biden, many with spouses, were pictured chatting to the royals.
After a 40-minute reception, they moved to an area to take a socially distanced group photo, and after taking their seats and posing for a few moments, the Queen said: "Are you supposed to be looking as if you're enjoying yourself?"
The leaders around the Queen laughed.
Mr Johnson, who is hosting the summit in Cornwall, added: "We have been enjoying ourselves - in spite of appearances."
Mrs Merkel walked over to the Queen and thanked her for posing for the picture, saying: "Thank you for doing that for us."
The monarchy does not wield power but has influence and uses what is known as "soft diplomacy" to help the UK renew and strengthen friendships with its allies and partners around the world.
This comes as Prince Charles urged G7 leaders to display the same sense of urgency in tackling climate change as they showed in tackling the coronavirus pandemic.
At a reception with leaders including Boris Johnson and Joe Biden, Charles said the Covid-19 pandemic had shown what a "truly borderless crisis" looks like.
"Of course, we did not fully see Covid coming," he said.
"Yet climate change and biodiversity loss represent a borderless crisis, the solutions to which have been argued about and postponed for far too long."
He said the battle against the pandemic provided a "crystal-clear example of the scale, and sheer speed, at which the global community can tackle crises when we combine political will with business ingenuity and public mobilisation".
Addressing the G7 leaders and a group of company bosses, the prince said: "We are doing it for the pandemic. So if you don't mind me saying so, we must also do it for the planet."
The prince has formed a group of more than 300 global business leaders to make a "coalition of the willing" in his Sustainable Markets Initiative.