"That said, to have a link like that with the EU is important," he concluded, in an apparent reference to his passport application.
His mother, known in the family as Granny Butter, claimed to come from an ancient line of French nobility.
Her own mother's maiden name was de Pfeffel, which was adopted as one of Boris Johnson's middle names.
The Prime Minister and his siblings spent part of their childhood in Brussels as their father worked for the European Commission.
Johnson senior was also the Tory MEP for Wight and Hampshire East from 1979 to 1984.
A source close to the family suggested he had applied so that his grandchildren could live and work in the EU after Brexit.
His plan to apply for a French passport was revealed in his daughter Rachel Johnson's book, Rake's Progress: My Political Midlife Crisis, when it was published in March.
Rachel Johnson wrote that her father was "en route to becoming a French citizen, as his mother had been born in Versailles and his grandmother had been in Paris".
"This is good news – I might be able to become French too," she added.
Johnson senior has been mulling over the application for some time. Asked about the suggestion in November 2019, he said it was a "nice thought".