The US President and his wife, Jill, enjoyed a stroll to the outdoor seating area of the Tregenna Castle Hotel in St Ives in Cornwall. Onlookers watched Mr Biden drink a Coca-Cola and the First Lady enjoy a glass of wine.
BBC reporter Jon Sopel, who was at the bar too, took to Twitter to tell his followers about the encounter.
He wrote: “A first in my career @POTUS comes and sits down at table in a bar next to me and orders a drink.
“Am trying to act nonchalant #G7.”
He also shared a video of the couple taking a seat as he was being moved along by security personnel.
In the video, Mr Biden said of Cornwall: “We are enjoying our walk. Well I’ll tell ya, this is so beautiful it doesn’t need the sun.
Mr Sopel wrote of the encounter: “Hope I’m not revealing state secrets but @POTUS drank coke @FLOTUS a glass of red.
“Not a beer or cider or pasty in sight #G7.”
President Biden met the Prime Minister on Thursday to work on an agreement about bolstering their defence ties.
Mr Johnson later hailed the vis-a-vis as a success, branding it a “breath of fresh air”.