The two were seen by Joanna Geary, who works for Twitter, leaving a restaurant in Harlem, New York.
Boris Johnson appeared to be proud to be with his friend.
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He, according to Ms Geary, pointed out that he was out with David Cameron.
The two were reportedly dining together at the popular Red Rooster restaurant.
Ms Geary told source: “I was walking home after work to my apartment in Harlem when I noticed a number of black SUVs parked up outside of Red Rooster, a popular soul food restaurant in Harlem.
“As I passed, a number of people in suits left the restaurant and I instantly recognised the famous mop of hair belonging to Boris Johnson.
“I said ‘hello Boris!’ And he said hello back, shook my hand and asked me why I was there. ‘I live here now, I moved in November’, I replied.
She shook Boris’ hand and then spoke to him . She explained: “He said, ‘well then, you’ll recognise my friend David’, and pointed to Cameron, who had followed in a separate group out the door.”
The two then reportedly left in separate SUVs.
Ms Geary said she spoke to someone who had dined in the restaurant, who told her: “Everyone realised they were important, but had no idea who they were … ‘They were just bopping along to the music like the rest of us,’ he said. I said: ‘Were they Dad-dancing?’ He laughed and said: ‘That’s about the size of it.’”