He previously served as the British Ambassador to France from 2016 to 2021[2] and as the Downing Street Chief of Staff under then prime minister David Cameron from 2010 to 2016.

New College contemporaries included Rageh Omaar, Steve Hilton and Ian Katz.

Cameron became Prime Minister and appointed Llewellyn to the post of Downing Street Chief of Staff.

In July 2011, several newspapers reported that Llewellyn asked Metropolitan Police Assistant Commissioner John Yates not to speak to Cameron about the News International phone hacking scandal.

[17] Llewellyn was succeeded as ambassador by Menna Rawlings in summer 2021, but it was announced he would transfer to another Diplomatic Service appointment.