He engaged in business with Edward Livingston, who was then district attorney and mayor of New York. After Livingston moved to New Orleans, Duer formed a professional partnership with his brother-in-law, Beverley Robinson. In addition to practicing law, he contributed to a partisan weekly paper called the "Corrector", conducted by Peter Irving in support of Aaron Burr. Duer later moved to New Orleans to again form a law partnership with Livingston, and also made a study of Spanish civil law, which had governed New Orleans in its early years. Duer found the climate in New Orleans disagreeable to his health, and this factor coupled with his marriage with the daughter of William Denning, a prominent New York politician and businessman, convinced him to return to New York City.