He studied law, became a judge in the tribunal of the Seine in 1806, was attached to the cabinet of Louis Bonaparte in 1807, and was counsel to the court of appeal at Paris in 1811. Immediately upon the fall of the empire be declared himself a Royalist, and remained faithful to the Bourbons through the Hundred Days. He made the personal acquaintance of Louis XVIII during that period through Baron Louis, and the king rewarded his energy and tact by appointing him prefect of police at Paris on July 7 1815. His marked success in that difficult position won for him the ministry of police, in succession to Fouché, on September 24.
In the interval he had been elected deputy for the Seine (August 1815) and both as deputy and as minister he led the moderate Royalists. His formula was "to royalize France and to nationalize the monarchy." The Moderates were in a minority in the chamber of 1815, but Decazes persuaded Louis XVIII to dissolve the house, and the elections of October 1816 gave them a majority. During the next four years Decazes was called upon to play the leading role in the government.
At first, as minister of police he had to suppress the insurrections provoked by the ultra Royalists (the White Terror); then, after the resignation of the duc de Richelieu, he took the actual direction of the ministry, although the nominal president was General JJPA Dessolle (1767-1828). He held at the same time the portfolio of the interior. The cabinet, in which Baron Louis was minister of finance, and Marshal Gouvion Saint Cyr remained minister of war, was entirely Liberal; and its first act was to suppress the ministry of police, as Decazes held that it was incompatible with the régime of liberty. His reforms met with the strong hostility of the Chamber of Peers, where the ultra-Royalists were in a majority, and to overcome it he got the king to create sixty new Liberal peers.
He then passed the laws on the press, suppressing the censorship. By reorganization of the finances, the protection of industry and the carrying out of great public works, France regained its economic prosperity, and the ministry became popular. But the powers of the Grand Alliance had been watching the growth of Liberalism in France with increasing anxiety. Metternich especially ascribed this mainly to the "weakness" of the ministry, and when in 1819 the political elections still further illustrated this trend, notably by the election of the celebrated Abbé Grégoire, it began to be debated whether the time had not come to put in force the terms of the secret treaty of Aix-la-Chapelle. It was this threat of foreign intervention, rather than the clamour of the "Ultras," that forced Louis XVIII to urge a change in the electoral law that should render such a "scandal" as Grégoire's election impossible for the future.
Dessolles and Baron Louis, refusing to embark on this policy, now resigned; and Decazes became head of the new ministry, as president of the council (November 1819). But the exclusion of Grégoire from the chamber and the changes in the franchise embittered the Radicals without conciliating the "Ultras." The news of the revolution in Spain in January 1820 added fuel to their fury; it was the foolish and criminal policy of the royal favourite that had once more unchained the demon of revolution. Decazes was denounced as the new Sejanus, the modern Catiline; and when, on February 13, the duke of Berry was murdered, clamorous tongues loudly accused him of being an accomplice in the crime. Decazes, indeed, foreseeing the storm, at once placed his resignation in the king's hands. Louis at first refused. "They will attack," he exclaimed, "not your system, my dear son, but mine." But in the end he was forced to yield to the importunity of his family (February 17); and Decazes, raised to the rank of duke, passed into honourable exile as ambassador to Great Britain.
This ended Decazes's meteoric career of greatness. In December 1821 he returned to sit in the House of Peers, when he continued to maintain his Liberal opinions. After 1830 he adhered to the monarchy of July, but after 1848 he remained in retirement. He had organized in 1826 a society to develop the coal and iron of the Aveyron, and the name of Decazeville was given in 1829 to the principal centre of the industry.