In 1893 Viviani was elected to the Chamber of Deputies. Along with Jean Jaurés and Aristide Briand he was involved in the establishment of the French Socialist Party. In 1904 he helped form the left-wing newspaper, L'Humanité.
Viviani and Aristide Briand left the party in order to serve in the government established by Georges Clemenceau in October 1906. Viviani served as minister of labour and Briand as minister of public instruction and worship. Viviani remained in the cabinet when Briand became prime minister and did not leave office until November 1910.
Viviani returned to power in December 1913 as minister of education. The following year the French president, Raymond Poincare appointed Viviani as prime minister and minister of foreign affairs. His attempts to establish political control over the military high command during the First World War ended in failure and he was unable to persuade Joseph Joffre, chief of general staff in the French Army, to allow government inspections of the Western Front.
Viviani resigned as prime minister in October 1915, after being criticized for a munitions shortage. However, he continued in the government as minister of justice.
After the war Viviani was the French representative of the League of Nations. Rene Viviani died in Le Plessis-Robinson on 7th September, 1925.