Le Deunff obtained a master's degree in international economy and a doctorate in economics at the University of Paris (Nanterre). From 1981 to 1984, Le Deunff was a teacher of economics at the University of Paris. In 1985 and 1986, he taught Economics at the French Lycée of Montevideo, Uruguay.
From 1987 to 1991, Le Deunff was attaché at the French Consulate in San Francisco. He served the local French community and also assisted the French Consul General in various special projects, involving relations with the universities of Berkeley and Stanford.
In 1992 and 1993, Le Deunff worked as press attaché at the Consulate General of France in Stuttgart, Germany. In 1994, he joined the team of the Spokesman of the Ministry of Foreign Affairs in Paris. In this capacity, he was spokesperson of the French Ministry for matters regarding Central and Eastern Europe, the Balkans, international strategic affairs, and issues related to French expatriates.
From 1997 to 2000, Le Deunff was posted as First Secretary at the French Embassy in Kuala Lumpur, Malaysia. During the Asian financial crisis, his duties included the monitoring of the Malaysian political situation and the strengthening of political ties between France and Malaysia.
Between September 2000 and August 2003, he was First Secretary and Deputy Head of Mission at the French Embassy in Ljubljana, Slovenia. In this capacity, Le Deunff contributed to finalizing the Accession Treaty negotiation between Slovenia and the European Union.
Returning to Paris in September 2003, Le Deunff became adviser to Noelle Lenoir, French Minister of European Affairs, in charge of EU enlargement and EU external relations. After working with the Secretary General of the Ministry of Foreign Affairs, in April 2004 he was appointed deputy head of the Department for the EU Foreign and Security Policy at the MFA. He was responsible for formulating the French official position in all matters related to the European foreign policy.
From August 2006 until August 2009, Le Deunff was Senior Adviser and Deputy Head of Mission at the French Embassy in Bangkok, Thailand. While closely monitoring the local political turbulence and challenging regional issues, especially involving Burma and Cambodia, he also managed several consular and humanitarian crises for the benefit of the local and French communities.
From September 2009 until August 2012, Le Deunff was Consul General of France in Atlanta (United States), covering the States of Alabama, Georgia, Mississippi, North Carolina, South Carolina and Tennessee.
Le Deunff is married to Diana, who is Franco-American. In January 2009, he was awarded the silver Medal of Honour of the French Ministry of Foreign and European Affairs. In November 2009, he was appointed a Knight of the National Order of Merit by the President of the French Republic.