On November 29, 1887, he married Edith Redfield in New York. Edith's father Amasa A. Redfield was a New York attorney and author.
In 1888, he was admitted to the New York Bar, but promptly abandoned the practice of law. Returning to Columbia, he obtained an A.M. in 1891, serving as an associate instructor of Latin until 1894. In 1895, Columbia awarded him a Ph.D. and he became an associate professor of Latin and Sanskrit at New York University until 1902.
Professor Cooper was the editor of various periodicals, including The New York Commercial Advertiser (1898-1904), The Forum (1907-1909), and for a short time of the New York Globe. He died in New London, Connecticut, shortly after returning from a trip to Europe on May 20, 1937.
Word formation in the Roman Sermo Plebeius. An historical study of the development of vocabulary in vulgar and late Latin, with special reference to the Romance languages. Ph. D. thesis, Columbia College. New York (1895)
History of the Nineteenth Century in Caricature, with A. B. Maurice (1904)