Almost immediately upon his arrival in Louisiana, where the legal system had previously been based on Roman, French and Spanish law, and where trial by jury and other peculiarities of English common law were now first introduced, he was appointed by the legislature to prepare a provisional code of judicial procedure, which (in the form of an act passed in April 1805) was continued in force from 1805 to 1825. In 1807, after conducting a successful suit on behalf of a clients title to a part of the batture or alluvial land near New Orleans, Livingston attempted to improve part of this land (which he had received as his fee) in the Batture, Ste 1~Iarie. Great popular excitement was aroused against him; his workmen were mobbed; and Governor Claiborne, when appealed to for protection, referred the question to the Federal government. Livingstons case was damaged by President Jefferson, who believed that Livingston had favored Burr in the presidential election of i8oo, and that he had afterwards been a party to Burrs schemes. Jefferson made it impossible for Livingston to secure his title, and in 1812 published a pamphlet for the use of counsel in the case against Livingston, to which Livingston published a crushing reply. Livingstons final victory in the courts brought him little financial profit because of the heavy expenses of the litigation. During the war with England from 1812 to 1815 Livingston was active in rousing the mixed population of New Orleans to resistance. He used his influence to secure amnesty for Lafitte and his followers upon their offer to fight for the city, and in 1814-1815 acted as adviser and volunteer aide-de-camp to General Jackson, who was his personal friend. In 1821, by appointment of the legislature, of which he had become a member in the preceding year, Livingston began the preparation of a new code of criminal law and procedure, afterwards known in Europe and America as the Livingston Code. It was prepared in both French and English, as was required by the necessities of practice in Louisiana, and actually consisted of four c descrimes and punishments, procedure, evidence in criminal cases, reform and prison discipline. Though substantially completed in I8~4, when it was accidentally burned, and again in 1826, it was not printed entire until 1833. It was never adopted by the state. It was at once reprinted in England, France and Germany, attracting wide praise by its remarkable simplicity and vigour, and especially by reason of its philanthropic provisions in the code of reform and prison discipline, which noticeably influenced the penal legislation of various countries. In referring to this code, Sir Henry Maine spoke of Livingston as the first legal genius of modern times (Cambridge Essays, 1856, p.17). The spirit of Livingstons code was remedial rather than vindictive; it provided for the abolition of capital punishment and the making of penitentiary labor not a punishment forced on the prisoner, but a matter of his choice and a reward for good behaviour, bringing with it better accommodations. His Code of Reform and Prison Discipline was adopted by Guatemala. Livingston was the leading member of a commission appointed to prepare a new civil code, which for the most part the legislature adopted in 1825, and the most important chapters of which, including all those on contract, were prepared by Livingston alone.

Livingston was again a representative in Congress during i Preliminary work in the preparation of anewcivilcodehad been done by James Brown and Moreau Lislet, who in i8o8 reported a Digest of the Civil Laws now in force in the Territory of Orleans with Alterations and Amendments adapted to the present Form 01 Government 1823-1829, a senator in 1829f831, and for two years(I831I833) secretary of state under President Jackson. In this last position he was oneof the most trusted advisers of the president, for whom he prepared a number of state papers, the most important being the famous anti-nullification proclamation of the 10th of December 1832. From 1833 to 1835 Livingston was minister plenipotentiary to France, charged with procuring the fulfilment by the French government of the tfeaty negotiated by W. C. Rives in 1831, by which France had bound herself to pay an indemnity of twenty-five millions of francs for French spoliations of American shipping chiefly under the Berlin and Milan decrees, and the United States in turn agreed to pay to France 1,500,000 francs in satisfaction of French claims. Livingstons negotiations were conducted with excellent judgment, but the French Chamber of Deputies refused to make an appropriation to pay the first instalment due under the treaty in 1833, relations between the two governments became strained, and Livingston was finally instructed to close the legation and return to America. He died on the 23rd of May 1836 at Montgomery Place, Dutchess county, New York, an estate left him by his sister, to which he had removed in 1831. Livingston was twice married. His first wife, iviary McEvers, whom he married on the 10th of April 1788, died on the I3thof March 1801. InJune 1805 he married Madame Louise Moreau de Lassy (d. 1860), a widow nineteen years of age, whose maiden name was Davezac de Castera, and who was a refugee in New Orleans from the revolution in Santo Domingo. She was a woman of extraordinary beauty and intellect, and is said to have greatly influenced her husbands public career.