In December 1943 Cooper was appointed British Representative on the Free French Committee of National Liberation (FCLN). His remit included maintaining a working relationship between Churchill and de Gaulle, two men whom he found equally difficult. Paris was liberated in August 1944 and he moved there in September. On 18 November 1944 he formally presented his credentials as British Ambassador to France He was to prove a very popular ambassador, with Lady Diana helping to make his term of office a great social success. Some contemporary eyebrows were raised at his willingness to entertain people with dubious records during the recent war, or his lack of interest at entertaining trade unionists. In the words of the British historian P. M. H. Bell, Cooper was such a "devoted Francophile" that during his time as ambassador to Paris he often tried the patience of the Foreign Office by going well beyond his instructions to maintain good relations with France by trying to create an Anglo-French alliance that would dominate post-war Europe.