He was born in Nantes, Loire-Atlantique of a bourgeois family. He attended the Nantes Lycée, where, in 1877, he developed a close friendship with Jules Verne. He studied law, and soon went into politics, associating himself with the most advanced movements, writing articles for the anarchist journal Le Peuple, and directing the Lanterne for some time. From this he passed to the Petite République, leaving it to found L'Humanité, in collaboration with Jean Jaurès.
At the same time he was prominent in the movement for the formation of trade unions, and at the congress of working men at Nantes in 1894 he secured the adoption of the labor union idea against the adherents of Jules Guesde. From that time, Briand was one of the leaders of the French Socialist Party. In 1902, after several unsuccessful attempts, he was elected deputy. He declared himself a strong partisan of the union of the Left in what was known as the Bloc, in order to check the reactionary Deputies of the Right.
From the beginning of his career in the Chamber of Deputies, Briand was occupied with the question of the separation of church and state. He was appointed reporter of the commission charged with the preparation of the 1905 law on separation, and his masterly report at once marked him out as one of the coming leaders. He succeeded in carrying his project through with but slight modifications, and without dividing the parties upon whose support he relied.
He was the principal author of the law of separation, but, not content with preparing it, he wished to apply it as well. The ministry of Maurice Rouvier was allowing disturbances during the taking of inventories of church property, a clause of the law for which Briand was not responsible. Consequently he accepted the portfolio of Public Instruction and Worship in the Sarrien ministry (1906). So far as the Chamber was concerned his success was complete. But the acceptance of a position in a bourgeois ministry led to his exclusion from the Unified Socialist Party (March 1906). As opposed to Jaurès, he contended that the Socialists should co-operate actively with the Radicals in all matters of reform, and not stand aloof to await the complete fulfillment of their ideals.