He was born in Nantes, Loire-Atlantique of a petit 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 Syndicalist 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.

He became a freemason in the lodge Le Trait d'Union in July 1887 while the lodge didn't record his name in spite of his repeated requests. The lodge declared "unworthy" to him on 6 September 1889. In 1895 he joined the lodge Les Chevaliers du Travail that was established in 1893.

At the same time he was prominent in the movement for the formation of trade unions, and at the congress of workers 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, 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 the reporter of the commission charged with the preparation of the 1905 law on separation, and his 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. He himself was atheist.

Briand served as Minister of Justice under Clemenceau in 1908-9, before succeeding Clemenceau as Prime Minister on 24 July 1909, serving until 2 March 1911. In social policy, Briand’s first ministry was notable for the passage of a bill in April 1910 for workers' and farmers' pensions. That same year, compulsory sickness and old-age insurance was introduced for 8 million rural and urban workers. However, a law court decision in 1912 that questioned the legality of compulsion “enabled a large proportion of employers and workers to evade the law.”

Briand again served as Minister of Justice 1912-13 under the premiership of the rightwinger Raymond Poincaré (soon to become President of the Republic), before again becoming Prime Minister for a few months from 21 January 1913 until 22 March 1913.

On 27 November Briand proposed that Joffre be effectively demoted to commander-in-chief in northern France, with both he and Sarrail reporting to the War Minister, although he withdrew this proposal after Joffre threatened resignation. The Closed Session began on 28 November and lasted until 7 December. Briand had little choice but to make concessions to preserve his government, and in a speech of 29 November he promised to repeal Joffre's promotion of December 1915 and in vague terms to appoint a general as technical adviser to the government. Briand survived a confidence vote by 344-160 (six months earlier he had won a confidence vote 440-80).

Late in 1916 Roques had been sent on a fact-finding mission to Salonika after Britain, Italy and Russia had pushed for the dismissal of the theatre commander Sarrail. To Briand’s and Joffre’s surprise, Roques returned recommending that Sarrail be reinforced and that Sarrail no longer report to Joffre. Coming on the back of the disappointing results of the Somme campaign and the defeat of Romania, Roques’ report further discredited Briand and Joffre and added to the Parliamentary Deputies’ demands for a closed session. In November Ferry presented a report on the shortage of manpower. A secret session was held on 21 November about calling up the Class of 1918 followed by another a week later.

Nivelle's appointment caused great friction between the British and French high commands, after Lloyd George attempted to have Haig placed under Nivelle's command at the Calais Conference in January. Briand only reluctantly agreed to attend another allied conference in London (12–13 March 1917) to resolve the matter. Briand resigned as Prime Minister on 20 March 1917 as a result of disagreements over the prospective Nivelle Offensive, to be succeeded by Alex Andre Ribot.

The idea was to provide a framework to contain France's former enemy while preserving as much of the 1919 Versailles settlement as possible. The Briand plan entailed the economic collaboration of the great industrial areas of Europe and the provision of political security to Eastern Europe against Soviet threats. The basis was economic cooperation, but his fundamental concept was political, for it was political power that would determine economic choices. The plan, under the Memorandum on the Organization of a System of European Federal Union, was in the end presented as a French initiative to the League of Nations. With the death of his principal supporter, German foreign minister Gustav Stresemann, and the onset of the Great Depression in 1929, Briand's plan was never adopted but it suggested an economic framework for developments after World War II that eventually resulted in the European Union.

Briand returned to power in 1921. He supervised the French role in the Washington Naval Conference of 1921–22. Three factors guided the French strategy and necessitated a Mediterranean focus: the French navy needed to carry a great many goods, the Mediterranean was the axis of chief interest, and a supply of oil was essential. The primary goal was to defend French North Africa, and Briand made practical choices, for naval policy was a reflection of overall foreign policy. The Conference agreed on the American proposal that capital ships be limited to a ratio of 5 to 5 to 3 for the United States, Britain, and Japan, with Italy and France allocated 1.7 each. France's participation reflected its need to deal with its diminishing power and reduced human, material, and financial resources.

Briand's efforts to come to an agreement over reparations with the Germans failed in the wake of German intransigence, and he was succeeded by the more bellicose Raymond Poincaré. In the wake of the Ruhr Crisis, however, Briand's more conciliatory style became more acceptable, and he returned to the Quai d'Orsay in 1925. He would remain foreign minister until his death in 1932. During this time, he was a member of 14 cabinets, three of which he headed himself.

Aristide Briand received the 1926 Nobel Peace Prize together with Gustav Stresemann of Germany for the Locarno Treaties (Austen Chamberlain of the United Kingdom had received a share of the Peace Prize a year earlier for the same agreement).

A 1927 proposal by Briand and United States Secretary of State Frank B. Kellogg for a universal pact outlawing war led the following year to the Pact of Paris, aka the Kellogg–Briand Pact.

As foreign minister Briand formulated an original proposal for a new economic union of Europe. Described as Briand's Locarno diplomacy and as an aspect of Franco-German rapprochement, it was his answer to Germany's quick economic recovery and Future political power. Briand made his proposals in a speech in favor of a European Union in the League of Nations on 5 September 1929, and in 1930, in his "Memorandum on the Organization of a Regime of European Federal Union" for the Government of France.