A pupil of the École des Chartes, his thesis was called Vues sur l’histoire de l’organisation judiciaire en France du IXe au XIIIe siècle considérée au point de vue des juridictions extraordinaires and he got the diploma of "archiviste paléographe" in 1874.
Cohn emigrated to New York City on May 13, 1875, and from 1876 to 1884 was the American correspondent of La République française, then edited by Léon Gambetta, whom he had known in France, and whose political views he had adopted.
In March, 1882, Cohn was appointed tutor in French at Columbia College, and soon afterward made an instructor.
By a popular vote of the French residents of New York he was chosen to deliver the funeral oration on Gambetta in 1883 at Tammany Hall, and in 1885 was called from Cambridge, Massachusetts, for a similar purpose, upon the death of Victor Hugo.
While at Harvard he was temporary head of the French department in Wellesley College, and in 1888 and 1889 conducted a summer school of languages at Oswego, New York.