The couple attended a small Catholic church in St Ives in the morning of the final day of the G7 summit.
One parishioner, known only as Margaret, said it was “lovely” that the president and his wife joined their service at the Sacred Heart and St Ia Catholic Church.
Watch: Joe and Jill Biden take part in morning Mass
Another woman at the church on Sunday, Annie Fitzpatrick, said: “I think ‘gobsmacked’ is probably a very true word.”
“It’s quite amazing, we went into the church and they took some details from us and I thought this is a bit unusual,” the 58-year-old said.
“About 10 minutes into the service the doors opened up and President Biden and Dr Jill Biden walked in and just sat in the pew just across from me.”
Ms Fitzpatrick said Mr Biden “quietly got on with his prayer like everyone was doing.”
She added: “He looked around and said ‘peace be with you,’ and I was one of them so I’m delighted. I’m not sure I will ever get over this moment completely.”
The church sits in St Ives, which is not far from Carbis Bay, where they had been staying for the G7 summit.
Father Philip Dyson said he had not been given advance warning that the president and his wife would be joining them for the service.
“We realised people were coming with security so I thought maybe the president would be coming along so we just welcomed him,” he said.
He admitted he was slightly nervous while conducting the service.
“It’s the President of the United States of America,” he said. “It’s a great occasion to welcome him into our parish, into the church, and it’s lovely to know he made the time in his busy schedule in order to come to Mass.”
The priest added that he had managed to have a “quick word” with Mr Biden.
“I welcomed him to Cornwall and he said he was enjoying his time here and there were many serious matters they were discussing and just hope it’s going to come to fruition,” he said.
Leaders of the UK, US, France, Germany, Italy, Canada and Japan met at Carbis Bay in Cornwall for the three-day G7 summit.
After the summit ended on Sunday, Mr Biden travelled to meet Queen Elizabeth at Windsor Castle, saying afterwards the monarch had reminded him of his mother.
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Additional reporting by agencies