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.
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.’”