In revolutionary terms Cambacérès was a moderate. During the trial of Louis XVI he protested that the Convention did not have the power to sit as a court and demanded that the king should have due facilities for his defence. Nevertheless, when the trial proceeded, he voted with the majority which declared Louis to be guilty, but recommended that the penalty should be postponed until it could ratified by a legislative body.

In 1793 Cambacérès became a member of the Committee of General Defence, but was not a member of its famous successor, the Committee of Public Safety, until the end of 1794, after the Reign of Terror had ended. In the meantime he worked on much of the legislation of the revolutionary period. During 1795 he was also employed as a diplomat, and negotiated peace with Spain.

Cambacérès was considered too conservative to be one of the five Directors who took power in the coup of 1795, and finding himself in opposition to the Directorate he retired from politics. In 1799, however, as the Revolution entered a more moderate phase, he became Minister of Justice. He supported the coup of 18 Brumaire (in November 1799) which brought Napoléon Bonaparte to power as First Consul in a new regime designed to establish a stable constitutional republic.

The Code Napoléon
In December 1799 Cambacérès was appointed Second Consul under Bonaparte. He owed this appointment to his vast legal knowledge and his reputation as a moderate republican. His most important work during this period was the drawing up of a new Civil Law Code, later called the Code Napoléon, France's first modern legal code. The code was promulgated by Bonaparte (as Emperor Napoleon) in 1804. It was the work of Cambacérès and a commission of four lawyers.

The Code was a revised form of Roman law, with some modifications drawn from the laws of the Franks still current in northern France (Coutume de Paris). The Code was later extended by Napoleon's conquests to Italy, the Netherlands, Belgium, western Germany and Spain, and indirectly to the Spanish colonies in Latin America. Cambacérès's work has thus been enormously influential in European and American legal history. Versions of the Code are still in force in Québec and Louisiana.

The Code dealt with Civil Law; other codes ensued for Penal Law, criminal procedure, civil procedure.

Cambacérès and homosexuality
It is widely believed that Cambacérès used the Code Napoléon (or rather, the associated Penal Code) to decriminalise male homosexuality, and the fact that he was himself homosexual gives credibility to this belief. His sexual orientation was well-known, and he does not seem to have made any effort to conceal it. He remained unmarried, and kept to the company of other bachelors. Napoléon is recorded as making a number of jokes on the subject. During the Consulate, Bonaparte, Cambacérès and Third Consul Charles-François Lebrun were known as "Qui, Quae et Quod." (He, She and It).

Before the Revolution, sexual conduct had been regulated by mediaeval ecclesiastical law. When the National Constituent Assembly abolished ecclesiastical courts in 1791, it therefore in effect decriminalised male homosexuality, although it is not clear that this was its intention (a similar state of affairs occurred during the early years of the Russian Revolution).

The authors of the Code Napoléon had the option of reintroducing a law against male homosexuality (as was eventually done in the Soviet Union), but chose not to do so, presumably at least partly as a result of the influence of Cambacérès. In this sense Cambacérès can be credited with the decision to make decriminalisation a permanent fact of French law. Recent research by Michael Sibalis has shown, however, that Napoléonic officials could and did repress homosexuality using other laws, such as "offenses against public decency."

Sibalis argues that while officials of the Napoléonic regime disliked what they saw "crimes against nature," such offences were seldom actually tried as such in the courts. He concludes that despite police surveillance and harassment, "the Revolutionary and Napoléonic period was a time of relative freedom," partially anticipating "contemporary legal toleration."

Later career
Cambacérès disapproved of Bonaparte's accumulation of power into his own hands, culminating in the proclamation of the First French Empire on 19 May 1804. But he retained high office under Napoléon: Arch-Chancellor of the Empire and President of the Senate. He also became a prince of the Empire and in 1808 was made Duke of Parma (French: duc de Parme). His duchy was one of the twenty created as duché grand-fief (among 2200 noble titles created by Napoleon)—a rare hereditary honor, extinguished in 1824; even rarer, it was created in another part of the peninsula then Napoleon's own Kingdom of Italy.

Under Napoléon, as under the revolutionary regime, he was a force for moderation, opposing adventures such as the invasion of Russia in 1812. As Napoléon became increasingly obsessed with military affairs, Cambacérès became the de facto domestic head of government of France, a position which inevitably made him increasingly unpopular as France's economic situation grew worse. His taste for high living attracted hostile comment. Nevertheless he was given credit for the justice and moderation of his government, although the enforcement of conscription was increasingly resented towards the end of the wars.

When the Empire fell in 1814 Cambacérès retired to private life, but was recalled during Napoléon's brief return to power in 1815. After the restoration of the monarchy, he was in danger of arrest for his revolutionary activities, and for a time he was exiled from France. But the fact that he had opposed the execution of Louis XVI counted in his favour, and in May 1818 his civil rights as a citizen of France were restored. He was a member of the Académie française, and lived quietly in provincial France until his death in 1824.