He was born in Saint-Omer, Pas-de-Calais.

After a brilliant academic career at the University of Paris, where he was lauréat of the faculty of law, he rapidly made his mark at the bar. He was secretary of the conference of advocates and one of the founders of the Sociéte de legislation comparée. During 1875 and 1876 he was successively director of criminal affairs and secretary-general at the ministry of justice. In 1877 he entered politics, playing a conspicuous part on the committee of legal resistance during the Brogue ministry; in the following year he was returned to the chamber as a moderate republican member for Boulogne, in his native département of Pas-de-Calais.

His impassioned yet reasoned eloquence gave him an influence which was increased by his articles in the Parlement in which he opposed violent measures against the unauthorized congregations. He devoted himself especially to financial questions, and in 1882 was reporter of the budget. He became one of the most prominent republican opponents of the Radical party, distinguishing himself by his attacks on the short-lived Gambetta ministry. He refused to vote the credits demanded by the Ferry cabinet for the Tongking expedition, and helped Georges Clemenceau overthrow the ministry in 1885. At the general election of that year he was a victim of the Republican rout in the Pas-de-Calais, and did not re-enter the chamber till 1887.

After 1889 he sat for St Omer. His fear of the Boulangist movement converted him to the policy of "Republican Concentration," and he entered office in 1890 as foreign minister in the Freycinet cabinet. He had an intimate acquaintance and sympathy with English 'institutions,' and two of his published works - an address, Biographie de Lord Erskine (1866), and Etude sur l'acte du 5 avril 1873 pour l'etablissement d'une cour supreme de justice en Angleterre (1874) - deal with English law; he also gave a fresh and highly important direction to French policy by the understanding with Russia, which was declared to the world by the visit of the French fleet to Kronstadt in 1891, and which subsequently ripened into a formal treaty of alliance. He retained his post in Émile Loubet's ministry (February - November 1892), and on its defeat he became president of the council, retaining the direction of foreign affairs. The government resigned in March 1893 over the refusal of the chamber to accept the Senate's amendments to the budget. On the election of Félix Faure as president of the Republic in January 1895, Ribot again became premier and minister of finance. On 10 June he was able to make the first official announcement of a definite alliance with Russia. On 30 October the government was defeated on the question of the Chemin de fer du Sud, and resigned office.

The real reason of its fall was the mismanagement of the Madagascar expedition, the cost of which in men and money exceeded all expectations, and the alarming social conditions at home, as indicated by the strike at Carmaux. After the fall of Jules Méline's ministry in 1898 M. Ribot tried in vain to form a cabinet of "conciliation." He was elected, at the end of 1898, president of the important commission on education, in which he advocated the adoption of a modern system of education. The policy of the Waldeck-Rousseau ministry on the religious teaching congregations broke up the Republican party, and Ribot was among the seceders; but at the general election of 1902, though he himself secured re-election, his policy suffered a severe check.

He actively opposed the policy of the Combes ministry and denounced the alliance with Jean Léon Jaurès, and on 13 January 1905 he was one of the leaders of the opposition which brought about the fall of the cabinet. Although he had been most violent in denouncing the anti-clerical policy of the Combes cabinet, he now announced his willingness to recognize a new régime to replace the Concordat of 1801, and gave the government his support in the establishment of the Associations culturelles, while he secured some mitigation of the seventies attending the separation.

He was re-elected deputy for St. Omer in 1906. In the same year he became a member of the Académie française in succession to the duc d'Audiffret-Pasquier; he was already a member of the Academy of Moral and Political Science. In justification of his policy in opposition he published in 1905 two volumes of his Discours politiques.

Ribot was brought in as prime minister for a few days in June 1914 following the collapse of the Doumergue government, and returned to power again in March 1917, following the fall of Briand. Ribot's final ministry was during the most dismal part of the First World War, seeing the failure of the Nivelle Offensive and the famous mutiny of the French soldiers which followed. Dismissed in September and replaced by minister of war Paul Painlevé, Ribot continued as foreign minister for a month before resigning in October.

The main grammar school (lycée) in Saint Omer, the Lycée Alexandre Ribot, bears his name today.