A lawyer at Poughkeepsie, N.Y., from 1785 and a New York state legislator from 1790, Kent moved to New York City in 1793, when he was appointed the first professor of law at Columbia University. He became a justice of the New York Supreme Court in 1798, chief justice of that tribunal in 1804, and chancellor of the state Court of Chancery (then the highest judicial office in New York) in 1814.

Once more professor of law at Columbia (1823–26), Kent delivered lectures that he revised and elaborated as Commentaries on American Law, 4 vol. (1826–30). This work deals with international law; the U.S. Constitution and the federal system; the law of various U.S. states; personal rights; and the law of property, both real and personal. In content and in organization it bears little resemblance to its great English counterpart, Sir William Blackstone's Commentaries on the Laws of England, 4 vol. (1765–69).

As judge and commentator, Kent relied as much as possible on the old English law. When common-law precedent was inadequate or lacking, he turned to the civil (Roman-derived) law; in the United States, England, and elsewhere, common-law courts citing Kent perforce adopted many principles derived from civil law.