Daniel Mayer was born in France in 1909. He joined the Socialist Party in 1927 and worked for its daily newspaper, Le Populaire, where he wrote about social issues.

A close friend of Leon Blum, Mayer remained in France after Henri-Philippe Petain signed the armistice with Germany on 22nd June, 1940. He joined the French Resistance and in January 1941 founded the Comité d'Action Socialiste. Mayer was soon joined by Pierre Brossolette, who ran a bookshop with his wife in Paris. In April 1942, Brossolette was sent to London to have talks with General Charles De Gaulle who was keen to unite these different resistance groups under his leadership.

Jean Moulin, who had spent time in London with De Gaulle, was sent back to France in 1942 and was given the task of uniting the various groups into one organization. He arranged meetings with people such as Mayer, Henry Frenay (Combat), Emmanuel d'Astier (Liberation-sud), Jean-Pierre Lévy (Francs-Tireur), Pierre Villon (Front National), Charles Tillon and Pierre Fabien (Frances-Tireurs Partisans) and Charles Delestraint (Armée Secrete).

After much discussion Jean Moulin persuaded the eight major resistance groups to form the Conseil National de la Resistance (CNR) and the first joint meeting under Moulin's chairmanship took place in Paris on 27th May 1943. Over the next year Mayer worked hard at persuading socialists and communists to support the unified resistance movement.

In August 1946 Mayer was replaced as general secretary of the Socialist Party. However, he did serve as a cabinet minister for three years (1946-49). Mayer was also president of the League of the Rights of Man (1958-75) and the Constitutional Council (1983-86). Daniel Mayer died in 1996.