During the quasi-war with France in 1798 he obtained the appointment of midshipman in the navy, and served under Stephen Decatur. After the conflict with France ended, he resumed his studies with Pendleton, and was admitted to the bar in 1802. 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. In New York, he contributed literary articles to the Morning Chronicle, the newspaper of his friend Peter Irving. He next opened an office in Rhinebeck, and in 1814 was elected to the New York State Assembly, where he was appointed chairman of a committee on colleges and academies, and succeeded in passing a bill, which is the original of the existing law on the subject of the common-school income. He was also chairman of the committee that arranged the constitutionality of the state law vesting the right of navigation in Livingston and Robert Fulton, and throughout his service bore a prominent part in promoting canal legislation. From 1822 until 1829, he was a judge of the New York Supreme Court. In 1829, he was elected president of Columbia College, where he remained until failing health compelled him to resign in 1842. During his administration he delivered to the senior class a course of lectures on the constitutional jurisprudence of the United States. He delivered a eulogy on President James Monroe from the portico of the city hall. After his retirement he resided in Morristown, New Jersey.
Works
After retiring, he wrote the life of his grandfather, Lord Stirling. In 1847 he delivered an address in the college chapel before the literary societies of Columbia, and in 1848 an historical address before the St. Nicholas Society, which gives early reminiscences of New York, and describes the scenes connected with the inauguration of President Washington, both of which were published. He was the author of two pamphlets addressed to Cadwallader D. Colden on the "Steamboat Controversy," and the "Life of William Alexander, Earl of Stirling".
Family
He was married to Hannah Maria Denning, daughter of William Denning, a prominent whig of New York. Together they had:
