Barrot was born at Villefort (Lozère). He belonged to a legal family, his father, an advocate of Toulouse, having been a member of the Convention who had voted against the death of Louis XVI. Odilon Barrot's earliest recollections were of the October insurrection of 1795. He was sent to the military school of Saint-Cyr, but later moved to the Lycee Napoleon to study law and was called to the Parisian bar in 1811. He was placed in the office of the conventionel Jean Mailhe, advocate before the council of state and the court of cassation, who was proscribed at the second restoration. Barrot eventually succeeded him in both positions. His dissatisfaction with the government of the restoration was shown in his conduct of some political trials.
For his opposition in 1820 to a law by which any person might be arrested and detained on a warrant signed by three ministers, he was summoned before a court of assize, but acquitted. Although intimate with Lafayette and others, he took no share in their schemes for the overthrow of the government, but in 1827 he joined the association known as "Aide-toi, le ciel t'aidera". He presided over the banquet given by the society to the 221 deputies who had signed the address of March 1830 to Charles X, and threatened to reply to force by force. After the ordinances of the 26th of July 1830, he joined the National Guard and took an active part in the revolution. As secretary of the municipal commission, which sat at the hôtel-de-ville and formed itself into a provisional government, he was charged to convey to the chamber of deputies a protest embodying the terms which the advanced Liberals wished to impose on the king to be elected. He supported the idea of a constitutional monarchy against the extreme Republicans, and he was appointed one of the three commissioners chosen to escort Charles X. out of France.
On his return he was nominated prefect of the department of the Seine. His concessions to the Parisian mob and his extreme gentleness towards those who demanded the prosecution of the ministers of Charles X led to an unflattering comparison with Jerome Pétion under similar circumstances. Louis Philippe's government was far from satisfying his desires for reform, and he persistently urged the "broadening of the bases of the monarchy," while he protested his loyalty to the dynasty. He was returned to the chamber of deputies for the department of Eure in 1831. The day after the demonstration of June 1832 on the occasion of the funeral of General Lamarque, he made himself indirectly the mouthpiece of the Democrats in an interview with Louis Philippe, which is given at length in his Mêmoires. Subsequently, in pleading before the court of cassation on behalf of one of the rioters, he secured the annulling of the judgments given by the council of war.
The death of the duke of Orleans in 1842 was a blow to Barrot's party, which sought to substitute the regency of the duchess of Orleans for that of the duke of Nemours in the event of the succession of the Comte de Paris. In 1846 Barrot made a tour in the Near East, returning in time to take part a second time in the preliminaries of revolution. He organized banquets of the disaffected in the various cities of France, and demanded electoral reform to avoid revolution. He did not foresee the strength of the outbreak for which his eloquence had prepared the way, and clung to the programme of 1830. He tried to support the regency of the duchess in the chamber on the 24th of February, only to find that the time was past for half-measures.
He acquiesced in the republic and gave his adhesion to General Cavaignac. He became the chief of Louis Napoleon's first ministry in the hope of extracting Liberal measures, but was dismissed in 1849 as soon as he had served the president's purpose of avoiding open conflict. After the coup d'etat of December 1851 he was one of those who sought to accuse Napoleon of high treason. He was imprisoned for a short time and retired from active politics for some ten years. He was drawn once more into affairs by the hopes of reform held out by Emile Ollivier, accepting in 1869 the presidency of an extraparliamentary committee on decentralization. After the fall of the empire he was nominated by Adolphe Thiers, whom he had supported under Louis Philippe, as president of the council of state. But his powers were failing, and he had only filled his new office for about a year when he died at Bougival.
He was described by Thureau-Dangin as "le plus solennel des indécis, le plus méditatif des irréfléchis, le plus heureux des ambitieux, le plus austere des courtisans de la foule." (Most solemn of the indecisive, most meditative of the unwise, happiest of the ambitious, most austere of the courtiers in the crowd.)