Cameron resigned in July after failing to keep Britain in the European Union in the June referendum.
But now his closest adviser Ed Llewellyn will play a diplomatic role in the negotiations for Britain's departure from the bloc as London's representative in Paris. Llewellyn will take up his post in Paris in December.
"He will help us make a great success of Brexit while ensuring that Britain is more engaged in the continent of Europe than ever," Foreign Secretary Boris Johnson said.
Llewellyn will succeed Julian King, who was recently appointed as the European commissioner for security on the European Union's executive body.
"I look forward to representing British interests and to building on the very strong relations our country has with France," Llewellyn said. "As we embark on a new chapter with the UK's departure from the EU, I will
work hard to strengthen our cooperation and to address the many common challenges facing our two nations."
Llewellyn, who attended the elite Eton College at the same time as Cameron, was the chief of staff throughout his time as leader of the centre-right Conservative Party, from 2005 to his resignation.
In his departing honours list, Cameron made Llewellyn a member of parliament's upper House of Lords. New Prime Minister Theresa May ditched many of Cameron's closest allies in her bid to create a clear break between her predecessor's administration and her own.
Her Downing Street office confirmed that she had approved the appointment.
Llewellyn is known in Hong Kong for his role as advisor to Chris Patten, the last governor of the British colony before it was handed over to China in 1997.