The responsibility for public health transfers on 1 April 2013 from the NHS to local authorities. Camden and Islington Councils anticipated closer collaboration to create a single Public Health Director to cover both authorities.
Both councils' chief executives were on the recruitment panel.
Julie Billet started her new role on 1 February 2013.
Camden chief executive, Mike Cooke, said: "We are very impressed by Julie's vision for our new joint service and her understanding of the challenges faced by both our councils over the coming years."
Islington chief executive, Lesley Seary added: "We are delighted Julie has started her new role as Director of Public Health for Camden and Islington.
"Julie's depth of knowledge and enthusiasm is impressive and her experience of delivering services will be a real asset in shaping and leading the public health team across our two boroughs."
Until 31 January 2013, Julie Billett was Deputy Director of Public Health in Islington, where she has worked for the past five years.